EPIDEMIOLOGY I
DEFINITION - EPIDEMIOLOGY
Epidemiology is the study of
the distribution and
determinants of disease or
health outcomes in specified
populations, and the
application of this study to the
control of health problems.
HIPPOCRATES
• Some people consider Hippocrates to be the first
epidemiologist. Hippocrates was a Greek physician.
He wrote a book called, On Airs, Waters, Places,
where he described epidemics. Hippocrates
thought about environmental and behavioral issues
that might be associated with disease. He focused
on rational explanations for disease rather than
supernatural explanations, which were the norm
during his time. For example, malaria was even
known about in ancient times.
HIPPOCRATES
• Hippocrates described the malarial parasite
periodicity and the malarial paroxysm. Which is a
period of intense chills, followed by a period of
intense fever, followed by a period of profuse
sweating, followed by exacerbation or flare-up.
Hippocrates was in favor of swamp drainage to
prevent miasma or bad night air. As he observed it's
link with yellow fever and malaria in people living
near these swamps.
• Introduced term, ”ENDEMIC” and “EPIDEMIC”
GIROLAMO FRACASTORO
• Italian physician Girolamo Fracastoro was another
contributor to the field of epidemiology. Fracastoro
showed several ways that transmission of infections
can occur. By direct contact, by air, and via
contaminated clothing. In 1546, Fracastoro
presented his writing on contagion and contagious
diseases, and proposed that diseases were each
caused by a rapidly multiplying seed. He proposed
that the seeds were transmitted by direct contact,
air, or contaminated garments.
JOHN GRAUNT
• John Graunt contributed to epidemiology as one of
the first demographers. Demography is the
statistical study of human populations. Graunt
calculated survivorship on a chart, and he
presented population and mortality statistics in
London during the time of the plague. The statistical
and census methods that he developed and used
became the basis for modern
• demography.
JOHN GRAUNT
• John Graunt did an analysis of the vital statistics of
people living in London. He published his analyses in
Natural and Political Observations Made Upon the
Bills of Mortality in 1662. This publication was the first
estimation of the London population that was
statistically-based.
JAMES LIND
• Lind was interested in the cause and
• treatment of scurvy. Sailors in the 16 and 1700s often
suffered from scurvy. Lind theorized that citrus fruits could
cure scurvy. Scurvy is now known to be caused by a
vitamin C deficiency. But back in the time of Lin, vitamins
were unknown. It is now known that vitamin C is
necessary to maintain healthy
• connective tissue. In the 1700s, during the
circumnavigation of the world by ship, many sailors died,
and many of those who died supposedly had scurvy.
Lind had the idea that something was lacking in the diet
of the ill sailors. Lind chose 12 men from a ship, all with
scurvy, and sorted them into six pairs. He gave each pair
a different supplement to their diet.
JAMES LIND
• All of the sailors received the same basic diet, but one
group received a quart of cider daily. Group two
received 25 drops of elixir of vitriol, or sulfuric acid. Group
three received six spoonfuls of vinegar. Group four
received half a pint of seawater. Group five received
two oranges and one lemon. And the last group
received a spicy paste. Group five could no longer be
treated after six days when they no longer had any fruit.
However, one sailor in this group was
• already able to work. While the other sailor had almost
progressed to the point that he too could soon return to
his post. Lind's work was a benefit to the field of
• preventative medicine, led to improved nutrition and
was a beginning step leading to modern day clinical
trials.
PERCIVALL POTT
• Percivall Pott was an English surgeon in the 1700s.
Pott is considered to be the first person to show that
an environmental carcinogen may cause cancer.
Pott noticed that London men working as
• chimney sweeps had more scrotal skin sores than
would be expected. He noticed coal soot in the
sores of the men he examined and concluded that
there was an association between men routinely
exposed to soot and scrotal cancer.
PERCIVALL POTT
• Pott's observations were the first time an
environmental factor was noted as a cancer
causing agent. Pott's work was the beginning of
modern non-infectious disease epidemiology.
WILLIAM FARR
• William Farr was a 19th century London
epidemiologist who's considered one of the
founders of
• modern epidemiology. Farr took statistical data and
tested social hypotheses. He also classified causes
of death in a way that accounted for broader
factors that determine health. Farr demonstrated a
relationship between population density and
mortality rates.
WILLIAM FARR
• He also mapped deaths, monitored
• outbreaks, and developed a new categorization system
for causes of death. The mortality data system that Farr
developed was an antecedent to the International
Classification of
• Diseases System that is used today. In 1849, there was a
major outbreak of cholera in London, in which 15,000
people died. This image is an extract from Farr's report
on mortality from cholera in the 1849
• outbreak. It was published in the Annual Report of the
Registrar General for 1854. Farr's report shows the
relationship between water sources and cholera. And
he even further classified cholera death by sub-districts
of different elevations.
SIR EDWIN CHADWICK
• Sir Edwin Chadwick studied sanitation issues in the
United Kingdom. Chadwick supported the idea that
disease was directly related to people's living
conditions and that there was a strong need for both
public health and social reform. Chadwick proposed
improvements such as fresh, clean water, water
closets, which was a room with a flushing toilet in each
home, and a sewage carrying system. Chadwick
promoted use a special, water type pipes for sewage
to reduce drinking water contamination. He thought
that improvements in the health of the poor would be
good for the nation overall. Chadwick published his
ideas in The Sanitary Condition of the Laboring
Population in 1842.
JOHN SNOW
• London physician John Snow, is known as The
Father of Modern Epidemiology. Snow
conducted the first outbreak investigation in
London in 1854. Snow showed an increase in
patients with cholera symptoms who lived or
worked in one district. Snow counted and
mapped cases of cholera according to where
people lived and worked. He noted that many
people who lived on or near Broad Street had
died. All had been ill with cholera symptoms.
JOHN SNOW
• Snow is likely the first person who
geographically counted and mapped
cases of a disease. He compared Cholera
in different parts of London and came to
the conclusion that people who used the
Broad Street water pump had much
higher rates of illness when compared to
people from other areas of London. Snow's
research showed that contaminated
water had spread cholera.
JOHN SNOW
• Snow's work is often considered the beginning of
epidemiology, the study of health outcomes in
populations. This figure shows a map of cholera
in London prepared by John Snow. By the end
of the outbreak, more than 600 people had
died from cholera. And all were exposed to
water from the Broad Street pump. This image
shows the Broad Street pump. Removing the
pump handle as a control measure that further
confirmed John Snow's hypotheses, as a number
of new cholera cases dropped. John Snow's
memory continues to live on today through the
John Snow Society.
JOHN SNOW
• The John Snow Society aims to promote the life
and works of Doctor John Snow, the pioneer of
the epidemiologic method. The John Snow
Society encourages members to celebrate the
memory of John Snow by visiting the John
Snow Pub, which is located on the site of the
original pump. This concludes our lecture on
the history of epidemiology. In the next lecture,
you'll learn about the definitions of
epidemiology, and how epidemiology fits into
public health.
NATURAL HISTORY AND
SPECTRUM OF DISEASE
Natural history of disease refers to the progress
of a disease process in an individual over
time, in the absence of intervention.
The process begins with exposure to or
accumulation of factors capable of causing
disease.
Without medical intervention, the process
ends with recovery ,disability, or death
Introduction to
epidemiology
Introduction to
epidemiology
NATURAL HISTORY AND
SPECTRUM OF DISEASE
Infectivity refers to the proportion of exposed
persons who become infected.
Pathogenicity refers to the proportion of infected
persons who develop clinical disease.
Virulence refers to the proportion of persons with
clinical disease who become severely ill or die
Introduction to
epidemiology
NATURAL HISTORY AND
SPECTRUM OF DISEASE
Hepatitis A virus in children has low Pathogenicity and
low virulence, since many infected children remain
asymptomatic and few develop severe illness.
In persons with good nutrition and health, measles virus
has high Pathogenicity but low virulence, since almost all
infected persons develop the characteristic rash illness
but few develop the life-threatening presentations of
measles, pneumonia, or encephalitis.
 In persons with poorer nutrition and health, measles is a
more virulent disease, with mortality as high as 5-10%.
Finally, rabies virus is both highly pathogenic and virulent,
since virtually 100% of all infected persons (who do not
receive treatment) progress to clinical disease and
death.
Introduction to
epidemiology
NATURAL HISTORY AND
SPECTRUM OF DISEASE
The natural history and spectrum of disease presents
challenges to the clinician and to the public health
worker. Because of the clinical spectrum, cases of
illness diagnosed by clinicians in the community
often represent only the “tip of the iceberg.” Many
additional cases may be too early to diagnose or
may remain asymptomatic.
Introduction to
epidemiology
NATURAL HISTORY AND
SPECTRUM OF DISEASE
For the public health worker, the challenge is that
persons with inapparent or undiagnosed infections
may nonetheless be able to transmit them to others.
Such persons who are infectious but have
subclinical disease are called carriers.
Introduction to
epidemiology
NATURAL HISTORY AND
SPECTRUM OF DISEASE
Frequently, carriers are persons with
incubating disease or inapparent infection.
Persons with measles, hepatitis A, and
several other diseases become infectious a
few days before the onset of symptoms. On
the other hand, carriers may also be
persons who appear to have recovered
from their clinical illness, such as chronic
carriers of hepatitis B virus
Introduction to
epidemiology
CHAIN OF INFECTION
Introduction to epidemiology
CHAIN OF INFECTION
The traditional model (epi triad) illustrates
that infectious diseases result from the
interaction of agent, host, and
environment. More specifically,
transmission occurs when the agent
leaves its reservoir or host through a
portal of exit, and is conveyed by some
mode of transmission, and enters through
an appropriate portal of entry to infect a
susceptible host. This is sometimes called
the chain of infection and is illustrated in
Introduction to
epidemiology
CHAIN OF INFECTION
Introduction to
epidemiology
CHAIN OF INFECTION
Reservoir: is the habitat in which an infectious
agent normally lives, grows, and multiplies.
Reservoirs include humans, animals, and the
environment. The reservoir may or may not be
the source from which an agent is transferred
to a host. For example, the reservoir of
Clostridium botulinum is soil, but the source of
most botulism infections is improperly canned
food containing C. botulinum spores.
Introduction to
epidemiology
CHAIN OF INFECTION
Human reservoirs. Many of the common
infectious diseases have human reservoirs.
Diseases which are transmitted from person to
person without intermediaries include the
sexually transmitted diseases, measles, mumps,
streptococcal infection, most respiratory
pathogens, and many others. Smallpox was
eradicated after the last human case was
identified and isolated because humans were
the only reservoir for the smallpox virus. Two
types of human reservoir exist:
 persons with symptomatic illness
 carriers
Introduction to
epidemiology
CHAIN OF INFECTION
• A carrier is a person without apparent disease
who is nonetheless capable of transmitting the
agent to others. Carriers may be asymptomatic
carriers, who never show symptoms during the
time they are infected, or may be incubatory or
convalescent carriers, who are capable of
transmission before or after they are clinically ill.
• A chronic carrier is one who continues to harbor
an agent (such as hepatitis B virus or Salmonella
typhi—the agent of typhoid fever) for a extended
time (months or years) following the initial
infection.
Introduction to
epidemiology
CHAIN OF INFECTION
Carriers commonly transmit disease because
they do not recognize they are infected
and consequently take no special
precautions to prevent transmission.
Symptomatic persons, on the other hand,
are usually less likely to transmit infection
widely because their symptoms increase
their likelihood of being diagnosed and
treated, thereby reducing their opportunity
for contact with others.
Introduction to
epidemiology
CHAIN OF INFECTION
Animal reservoirs. Infectious diseases that are
transmissible under normal conditions from animals
to humans are called zoonoses .In general, these
diseases are transmitted from animal to animal, with
humans as incidental hosts. Such diseases include
brucellosis (cows and pigs), anthrax (sheep),
plague (rodents), trichinosis (swine), and rabies
(bats, raccoons, dogs, and other mammals).
Introduction to
epidemiology
CHAIN OF INFECTION
Another group of diseases with animal reservoirs are
those caused by viruses transmitted by insects and
caused by parasites that have complex life cycles,
with different reservoirs at different stages of
development. Such diseases include St. Louis
encephalitis and malaria (both requiring mosquitos)
and schistosomiasis (requiring fresh water snails).
Lyme disease is azoonotic disease of deer
incidentally transmitted to humans by the deer tick.
Introduction to
epidemiology
CHAIN OF INFECTION
Environmental reservoirs. Plants, soil, and water in
the environment are also reservoirs for some
infectious agents. Many fungal agents, such as
those causing histoplasmosis, live and multiply in the
soil. The primary reservoir of Legionnaires’ bacillus
appears to be pools of water, including those
produced by cooling towers and evaporative
condensers.
Introduction to
epidemiology
CHAIN OF INFECTION
Portal of exit is the path by which an agent
leaves the source host. The portal of exit usually
corresponds to the site at which the agent is
localized. Thus, tubercle bacilli and influenza
viruses exit the respiratory tract, schistosomes
through urine, cholera vibrios in feces,
Sarcoptes scabiei in scabies skin lesions, and
enterovirus 70, an agent of hemorrhagic
conjunctivitis, in conjunctival secretions. Some
blood borne agents can exit by crossing the
placenta (rubella, syphilis, toxoplasmosis),
while others exit by way of the skin
(percutaneously) through cuts or needles
(hepatitis B) or blood-sucking arthropods
(malaria).
Introduction to
epidemiology
CHAIN OF INFECTION
Modes of transmission: After an agent exits its natural
reservoir, it may be transmitted to a susceptible host
in numerous ways. These modes of transmission are
classified as:
Direct
Direct contact
Droplet spread
Indirect
Airborne
Vehicleborne
Vectorborne
Mechanical
Biologic
Introduction to
epidemiology
CHAIN OF INFECTION
In direct transmission, there is essentially immediate
transfer of the agent from a reservoir to a
susceptible host by direct contact or droplet
spread. Direct contact occurs through kissing, skinto-skin contact, and sexual intercourse. Direct
contact refers also to contact with soil or
vegetation harboring infectious organisms.
Introduction to
epidemiology
CHAIN OF INFECTION
Infectious mononucleosis (“kissing disease”) and
gonorrhea are spread from person-to-person by
direct contact. Hookworm is spread by direct
contact with contaminated soil. Droplet spread
refers to spray with relatively large, short-range
aerosols produced by sneezing, coughing, or even
talking. Droplet spread is classified as direct
because transmission is by direct spray over a few
feet, before the droplets fall to the ground.
Introduction to
epidemiology
CHAIN OF INFECTION
In indirect transmission, an agent is carried
from a reservoir to a susceptible host by
suspended air particles or by animate
(vector) or inanimate (vehicle)
intermediaries. Most vectors are arthropods
such as mosquitoes, fleas, and ticks. These
may carry the agent through purely
mechanical means. For example, flies carry
Shigella on appendages; fleas carry Yersinia
pestis (agent that causes plague) in the gut
and deposit the agent on the skin of a new
host
Introduction to
epidemiology
CHAIN OF INFECTION
In mechanical transmission, the agent does not
multiply or undergo physiologic changes in the
vector. This is in contrast to instances in which an
agent undergoes part of its life cycle inside a vector
before being transmitted to a new host. When the
agent undergoes changes within the vector, the
vector is serving as both an intermediate host and a
mode of transmission. This type of indirect
transmission is a biologic transmission
Introduction to
epidemiology
CHAIN OF INFECTION
Guinea worm disease and many other
vectorborne diseases have complex life
cycles which require an intermediate host.
Since the agent undergoes part of its life
cycle in the intermediate host, the agent
cannot be transmitted by the intermediate
host until the agent has completed that part
of its life cycle. Therefore, this is an indirect,
vectorborne, biologic transmission.
Introduction to
epidemiology
CHAIN OF INFECTION
Vehicles that may indirectly transmit an agent
include food, water, biologic products (blood),
and fomites (inanimate objects such as
handkerchiefs, bedding, or surgical scalpels). As
with vectors, vehicles may passively carry an
agent—as food or water may carry hepatitis A
virus—or may provide an environment in which
the agent grows, multiplies, or produces toxin—
as improperly canned foods may provide an
environment in which C. botulinum produces
toxin.
Introduction to
epidemiology
CHAIN OF INFECTION
Airborne transmission is by particles that are
suspended in air. There are two types of these
particles: dust and droplet nuclei. Airborne dust
includes infectious particles blown from the soil by
the wind as well as material that has settled on
surfaces and become resuspended by air currents.
Introduction to
epidemiology
CHAIN OF INFECTION
Droplet nuclei are the residue of dried droplets.
The nuclei are less than 5 μ (microns) in size and
may remain suspended in the air for long
periods, may be blown over great distances,
and are easily inhaled into the lungs and
exhaled. This makes them an important means
of transmission for some diseases. Tuberculosis,
for example, is believed to be transmitted more
often indirectly, through droplet nuclei, than
directly, through droplet spread. Legionnaires’
disease and histoplasmosis are also spread
through airborne transmission.
Introduction to
epidemiology
CHAIN OF INFECTION
Portal of entry An agent enters a susceptible host
through a portal of entry. The portal of entry must
provide access to tissues in which the agent can
multiply or a toxin can act. Often, organisms use the
same portal to enter a new host that they use to
exit the source host. For example, influenza virus
must exit the respiratory tract of the source host and
enter the respiratory tract of the new host.
Introduction to
epidemiology
CHAIN OF INFECTION
The route of transmission of many enteric (intestinal)
pathogenic agents is described as “fecaloral”
because the organisms are shed in feces, carried
on inadequately washed hands, and then
transferred through a vehicle (such as food, water,
or cooking utensil) to the mouth of a new host.
Other portals of entry include the skin (hookworm),
mucous membranes (syphilis, trachoma), and
blood (hepatitis B).
Introduction to
epidemiology
CHAIN OF INFECTION
The final link in the chain of infection is a
susceptible host. Susceptibility of a host
depends on genetic factors, specified acquired
immunity, and other general factors which alter
an individual’s ability to resist infection or to limit
pathogenicity. An individual’s genetic makeup
may either increase or decrease susceptibility.
General factors which defend against infection
include the skin, mucous membranes, gastric
acidity, cilia in the respiratory tract, the cough
reflex, and nonspecific immune response.
Introduction to
epidemiology
CHAIN OF INFECTION
General factors that may increase susceptibility are
malnutrition, alcoholism, and disease or therapy
which impairs the nonspecific immune response.
Specific acquired immunity refers to protective
antibodies that are directed against a specific
agent. Individuals gain protective antibodies in two
ways: 1) They develop antibodies in response to
infection, vaccine, or toxoid; immunity developed
in these ways is called active immunity. 2) They
acquire their mothers’ antibodies before birth
through the placenta or they receive injections of
antitoxins or immune globulin; immunity that is
acquired in these ways is called passive immunity.
Introduction to
epidemiology
CHAIN OF INFECTION
Note that the chain of infection may be interrupted
when an agent does not find a susceptible host. This
may occur if a high proportion of individuals in a
population is resistant to an agent. These persons
limit spread to the relatively few who are
susceptible by reducing the probability of contact
between infected and susceptible persons. This
concept is called herd immunity.
Introduction to
epidemiology
CHAIN OF INFECTION
The degree of herd immunity necessary to prevent
or abort an outbreak varies by disease. In theory,
herd immunity means that not everyone in a
community needs to be resistant (immune) to
prevent disease spread and occurrence of an
outbreak. In practice, herd immunity has not
prevented outbreaks of measles and rubella in
populations with immunity levels as high as 85 to
90%
Introduction to
epidemiology
CHAIN OF INFECTION
One problem is that, in highly immunized
populations, the relatively few susceptible persons
are often clustered in population subgroups, usually
defined by socioeconomic or cultural factors. If the
agent is introduced into one of these subgroups, an
outbreak may occur.
Introduction to
epidemiology
CHAIN OF INFECTION
Implications for public health By knowing how an
agent exits and enters a host, and what its modes
of transmission are, we can determine appropriate
control measures. In general, we should direct
control measures against the link in the infection
chain that is most susceptible to interference, unless
practical issues dictate otherwise.
Introduction to
epidemiology
CHAIN OF INFECTION
For some diseases, the most appropriate
intervention may be directed at controlling or
eliminating the agent at its source. In the hospital
setting, patients may be treated and/or isolated,
with appropriate “enteric precautions,” “respiratory
precautions,” “universal precautions,” and the like
for different exit pathways. In the community, soil
may be decontaminated or covered to prevent
escape of the agent.
Introduction to
epidemiology
CHAIN OF INFECTION
Sometimes, we direct interventions at the mode of
transmission. For direct transmission, we may provide
treatment to the source host or educate the source
host to avoid the specific type of contact
associated with transmission. In the hospital setting,
since most infections are transmitted by direct
contact, handwashing is the single most important
way to prevent diseases from spreading.
Introduction to
epidemiology
CHAIN OF INFECTION
For vehicle borne transmission, we may
decontaminate or eliminate the vehicle. For
fecal-oral transmission, we may also try to
reduce the risk of contamination in the future
by rearranging the environment and educating
the persons involved in better personal hygiene.
For airborne transmission, we may modify
ventilation or air pressure, and filter or treat the
air. For vector borne transmission, we usually
attempt to control (i.e., reduce or eradicate)
the vector population.
Introduction to
epidemiology
CHAIN OF INFECTION
Finally, we may apply measures that protect portals
of entry of a susceptible potential host or reduce
the susceptibility of the potential host. For example,
a dentist’s mask and gloves are intended to protect
the dentist from a patient’s blood, secretions, and
droplets, as well to protect the patient from the
dentist. Prophylactic antibiotics and vaccination
are strategies to improve a potential host’s
defenses.
Introduction to
epidemiology
EPIDEMIOLOGIC TRANSITIONS.
• An epidemiologic transition describes changing
patterns of population age distributions, mortality,
fertility, life expectancy, and causes of death. The
human population has gone through four major
disease transitions since the agricultural period
began.
FIRST TRANSITION
• The first transition is related to the emergence of
infectious diseases and diseases related to nutrition.
And to the beginning of the practice of agriculture
and food production. 10,000 years ago, people
became less dependent on hunting and gathering
once they were able to produce food and
domesticate animals. This shift led to zoonotic
disease, nutrient deficiencies, and increased
contact with disease vectors during agricultural
activities.
SECOND TRANSITION
• The second transition was a time where human
immune systems and disease-causing organisms
both evolved, resulting in a change from major
epidemics of disease to endemic disease. People
developed physical and genetic changes that
serve to minimize the effects of diseases.
THIRD TRANSITION
• The third transition resulted when disease patterns
changed from infectious to chronic and
• degenerative diseases, in developed parts of the
world due to improvements in nutrition, public
health,
• and clinical medicine. Cardiovascular diseases and
cancer began to occur more during this transition.
They are often associated with the longer lifespan
and a sedentary lifestyle.
FOURTH TRANSITION
• The fourth transition started at the end of
the 20th century. This is the period when
both new diseases and the
reemergence of infectious ones
occurred as well as the rapid spread of
disease due to globalization. So let's
compare the four epidemiologic
transitions.
TRANSITION SUMMARY
• The first transition started when hunter gatherers began
to practice agriculture and began to live in one place.
There was a transition from few epidemics to major
epidemics.
• The second transition happened because populations
developed immunologic resistance and disease-causing
organisms changed. There was a transition from major
epidemics of disease to endemic diseases.
• The third transition came from improved public health
and sanitation. There was a transition from infectious
diseases to non-infectious chronic and degenerative
diseases.
• The fourth transition was due to increased globalization.
There was a resurgence and rapid spread of infectious
diseases.
EPIDEMIC DISEASE OCCURRENCE
Level of disease The amount of a
particular disease that is usually present in a
community is the baseline level of the disease.
This level is not necessarily the preferred level,
which should in fact be zero; rather it is the
observed level. Theoretically, if no intervention
occurred and if the level is low enough not to
deplete the pool of susceptible persons, the
disease occurrence should continue at the
baseline level indefinitely.
Introduction to
epidemiology
EPIDEMIC DISEASE OCCURRENCE
Thus, the baseline level is often considered the
expected level of the disease. For example, over
the past 4 years the number of reported cases of
poliomyelitis has ranged from 5 to 9. Therefore,
assuming there is no change in population, we
would expect to see approximately 7 reported
cases next year.
Introduction to
epidemiology
EPIDEMIC DISEASE OCCURRENCE

Different diseases, in different communities, show
different patterns of expected occurrence: a
persistent level of occurrence with a low to
moderate disease level is referred to as an
endemic level a persistently high level of
occurrence is called a hyperendemic level an
irregular pattern of occurrence, with occasional
cases occurring at irregular intervals is called
sporadic.
Introduction to
epidemiology
EPIDEMIC DISEASE OCCURRENCE
Occasionally, the level of disease rises above the
expected level. When the occurrence of adisease
within an area is clearly in excess of the expected
level for a given time period, it is called an
epidemic. Public health officials often use the term
outbreak, which means the same thing, because it
is less provocative to the public. When an epidemic
spreads over several countries or continents,
affecting a large number of people, it is called a
pandemic.
Introduction to
epidemiology
EPIDEMIC DISEASE OCCURRENCE
 Epidemics occur when an agent and susceptible
hosts are present in adequate numbers, and the
agent can effectively be conveyed from a source
to the susceptible hosts. More specifically, an
epidemic may result from the following:
1. A recent increase in amount or virulence of the
agent
2. The recent introduction of the agent into a setting
where it has not been before
3. An enhanced mode of transmission so that more
susceptibles are exposed
4. Some change in the susceptibility of the host
response to the agent
5. Factors that increase host exposure or involve
introduction through new portals of entry
Introduction to
epidemiology
EPIDEMIOLOGY AND PUBLIC HEALTH.
• Epidemiology has been used to evaluate the
success of immunization programs, such as the
one being implemented here in Mogadishu.
Public health is a multidisciplinary field whose
goal is to promote the health of the
population through organized community
efforts. Public health protects against threats
to health, empowers people to lead healthy
lives, improves the quality of health services.
And prepares leaders to advance health.
EPIDEMIOLOGY IN PH
• Two aspects of public health where
epidemiology is used. Are
• Prevention, such as raising public awareness
about the need for child car seats, and
• Interventions such as promoting healthy
eating by lowering the cost of health foods.
LEVELS OF PREVENTION
• Epidemiology is used, to develop, target, and
evaluate prevention strategies. There are three
levels of public health prevention. These are :
• Primary
• Secondary
• Tertiary
PRIMARY PREVENTION
• Primary prevention occurs before a person
gets a health outcome and tries to prevent
the health outcome from occurring. Primary
prevention may reduce the prevalence, risk
and, or rate of a health outcome. An
example of primary health prevention is
vaccination. Vaccines prevent diseases
from being contracted.
SECONDARY PREVENTION
• Secondary prevention occurs after a
disease has occurred, but before the person
notices symptoms. Secondary prevention
aims to find and treat disease early, which
may lead to the disease being cured. An
example of a secondary health prevention
is the doctor checking for suspicious skin
growths, in order to detect and treat skin
cancer early.
TERTIARY PREVENTION
• Tertiary prevention refers to a person who
already has symptoms of the disease. The
goal of tertiary prevention is to prevent
damage and pain from disease. Slow down
disease progression, prevent complications,
provide better care and help people
recover. An example of a tertiary prevention
would be offering dietary advice to help
manage the symptoms of diabetes.
EPIDEMIOLOGY AND INTERVENTION
• Epidemiology is also used to evaluate
interventions in the population. Public
health interventions can occur at two
levels,
• Individual or
• Structural.
INDIVIDUAL INTERVENTIONS
• Individual interventions focus on changing individual
risk factors or behavior. Individual intervention
approaches rely on the assumption that people have
the autonomy and capacity to make choices and
then follow through on their choices. An example of an
individual intervention, might be educating individuals
on the health benefits of quitting smoking. Structural
interventions focus on changing physical, social,
and/or economic factors in the environment to
improve public health. Provision of clean drinking
water.
INDIVIDUAL
• Improving road safety standards, changes in car
designs or changes in urban design to increase
physical activity are examples of structural
interventions. In contrast to an individual
intervention, a structural intervention to prevent
smoking, might promote taxation of tobacco
products to make them less affordable.
STRUCTURAL INTERVENTIONS
• Structural interventions rely on the assumption that
public health problems have some social causation.
Public health is preventing illness and disability and
prolonging life. Promoting physical and mental
health and efficiency. Community efforts for
sanitation of the environment. Prevention of
diseases and injuries. Education of the individual.
Organization of services for the diagnosis and
treatment of disease. And promoting a standard of
living adequate for the maintenance of health.
WHAT DOES EPIDEMIOLOGY
MEASURES
• Epidemiology looks at the distribution of a disease in a
given group
• Involves the use of numbers( Biostatistics)
• Investigates the number of actual cases with the number
of potential cases to determine the rate
RATE = ACTUAL CASES
POTENTIAL CASES
Numerator
Denominator
• Denominator. The group who is a risk for a particular event
or disease
INCIDENCE RATE
• The rate at which new events occur in a
population usually over a one year period
Incidence rate=
num. of new events
num. of individual who
can be exposed to
the event
• Attack Rate- a form of incidence rate in which
the denominator is reduce further for some known
exposure ( Acute conditions)
• Prevalence Rate - all the person in a population
who experience an event ( illness)
Prevalence rate =
num. of person with illness
total population at risk
• Point prevalence = the num. of individual who
have an illness at a particular time. E.g. : the num.
of individual with cancer at the end Oct. divided
by the total population at that time
PREVALENCE (CONT)
• Period Prevalence = num. of
individual who have an illness at a
specific time period.
e.g. the num. of cancer in 2007
divided the total population mid year
in 2007
THE RELATIONSHIP
PREVALENCE AND INCIDENCE
• Prevalence = incidence rate multiplied by
the average duration of the disease
• Applied when both incidence and
duration are stable
• Prevalence is greater than incidence if the
disease is long tem.
Factor
Increasing survival
rates for disease
Rapid Recovery
Incidence
Prevalence
NO CHANGE
INCREASE
NO CHANGE DECREASED
Reduce risk
factors
DECREASED DECREASED
Vaccine
DECREASED DECREASED
Death from
condition
NO CHANGE
DECREASED
Effective treatment
initiated
NO CHANGE
DECREASED
CRUDE, SPECIFIC AND STANDARDIZED
RATES
• Crude rate – measured rate of the whole population
• Specific Rate - measured rate for sub group of
population e.g.. Age , sex, race, religion
• Standardized rate – adjust or remove any difference
between two population based on the standardize
variable
Allows for uncontaminated comparison
Adjusted to make group equal on the same form factor.
• Crude Mortality rate -
num. of deaths
total population
• Cause specific mortality rate – death from cause
Total population
• Case Fatality Rate –
Death from Cause
num. of person with the disease
• Proportionate Mortality Rate(PMR) –Death from cause
All deaths
SCREENING TESTS
Sensitivity – asses the individuals who actually
have the disease under investigation.
True Positives- individuals whom the test has
correctly identified as being ill
False Positives- individuals whom the test
has incorrectly identified as being ill.
VALIDITY
Disease State
Research
Decision
Total
Total
Present
Absent
+VE
True
Positive
False
Positive
TP + FP
-VE
False
Negativ
e
True
Negativ
e
FN + TN
TP + FN
FP + TN
SENSITIVITY
• Sensitivity =
True Positives
(True Positives+ False Negatives)
SPECIFICITY
Specificity/ true negative rate- asses how
well a test identify truly well individuals
True Negatives - individuals who are well
whom the test has identified as truly being
well
False Positives - well individuals whom the
test has incorrectly identified as being ill
SPECIFICITY
• Specificity =
True Negatives
(True Negatives + False Positives)
PREDICTIVE VALUE
• Predictive Value - measure the percentage of
test result that match the actual value.
Positive Predictive value- the probability that
someone with a positive test actually has the illness
+ predictive Value =
True Positive
(True Positive + False Positives)
• Negative predictive value – the probability of no disease
in a person who receives a negative test result
The person with a negative test is a true negative
True negatives
(True negatives + False Negatives)
Looks at the distribution of persons who receive a negative
test result
ACCURACY
• It is the total percentage correctly selected,
• Identify the degree to which a measurement or
an estimate is based on the measurement.
• (TP+TN)/(TP+TN+FP+FN) I.E Total screened patient
EPIDEMIOLOGIC STUDY DESIGNS
• The two major type of studies in epidemiology are:
1. Experimental study designs and
2. Observational study designs.
• Though experimental study is the gold standard for
causal relationship, sometimes we need to use
observational study designs because there's some
types of exposures that cannot be randomized, it's
not ethical.
OBSERVATION STUDIES
• The four major types of observation
studies are:
1.
2.
3.
4.
Cohort Studies
Case Control Studies
Cross sectional studies
Ecology Studies
COHORT STUDIES
• The cohort study provides the foundation for
understanding other types of observational
study designs. With the cohort study design,
researchers follow an at-risk study population
over time, and evaluate exposures over time,
and determine the subsequent risk or rate of
disease or health outcome.
• Some examples of a common characteristic
are geographic location, occupation,
socioeconomic status, age, gender, or race
or ethnicity.
RESEARCH STUDY DESIGN
• Cohort Studies:
-group who are free from condition under investigation
-exposure to risk factors linked to cause of illness
-assessment of incidence rate between exposed and non
exposed group
Types:
a) Historical
b) Prospective
TYPES OF COHORT
• There are two types of cohort studies, retrospective and
prospective. And they are classified according to their
temporal sequence.
• Retrospective and prospective refer to the time the
investigator initiates the study and starts collecting data.
Both designs assemble cohorts on the basis of exposure
first. In the retrospective study, the cohort is formed in the
past. The prospective study starts now and goes into the
future.
• In a prospective study, the investigators obtain
baseline exposure data in real time, and then
follow the cohort members during the time after
baseline exposure to measure the occurrence
of the health outcome or disease.
COHORT STUDIES BASICS
Cohort: Male and Female British Physicians
Exposure: Smoking
Base line
Exposure
1950
1960
Exposure
1970
Exposure
1980
1990
Development
of HO
2000
2010
2011
CROSS SECTIONAL STUDIES
• Like cohort studies, cross sectional studies
conceptually begin with a population base within
which the occurrence of disease or health outcome
and sometimes the simultaneous occurrence of the
exposure will be studied. For example, the population
could be all individuals currently living in Harari,
Ethiopia. Or it could be all children ages five to six
currently attending kindergarten in Seattle,
Washington. Or it could be all taxi drivers currently
working in Beijing, China. A key aspect of a cross
sectional study is that the exposure and the outcome
are assessed at the same point in time within the
specified study population.
• Cross Sectional Studies:
- Information is collected from a group of
individuals who provide a picture of the disease
over time
- Provide info on the relationship between risk
factors and health status of a group of individual
at one specific point in time
CASE CONTROL STUDIES
• Case control studies are an efficient and
common epidemiologic study design to study
rare diseases. The rule of thumb that we will be
using for this MOOC is, rare is defined as a
prevalence of less than 10%. In a case control
study, researchers begin by selecting diseased
individuals or individuals with a health
outcome of interest.
• Case Control studies
- Subjects with specific disorder i.e. the case
- Subjects without disease i.e. control
- Exposure to risk factors are assessed
ANALYZING STUDIES
• Relative risk – compares the incidence rate of a disorder
among individuals exposed to a risk factor
e.g. : incidence rate of colon cancer among smoker is
20/1000, the incidence rate of colon cancer is 2/1000
The chance of eithers is 20/1000 divided by 2/1000 = 10
Means that a smoker is 10 times at risk of getting colon ca
than non smoker
• Attributable Risk
Used for determining what would happen in
a study population if the risk factors were
removed
Illness incidence rate in non exposed
individuals is subtracted from the incidence
rate of the illness in those who have been
exposed to risk factor
BIAS IN STUDIES
Bias used to construct a study so that one outcome is
more likely to occur than another
Types:
Selection Bias
Measurement Bias
Lifetime Bias
Experimenter Expectancy
Recall Bias
Late look Bias
Confounding Bias
Design Bias
Sampling Bias
Selection Bias
The selected sample is not representative of the population
Can occur if the subjects are permitted to chose whether
which group to go into or
The investigator purposely chooses which patient go into
which group
Types:
Berkson Bias A form of selection bias that causes hospital cases and
controls in a case control study to be systematically different
from one another because the combination of exposure to
risk and occurrence of disease increases the likelihood of
being admitted to the hospital.
Non respondent Bias
Solution: random, independent data
Measurement Bias
Information is gathered in a manner
that distort the information
Hawthrone Effect occurs when
subject’s behavior is altered because
they are being study
Solution– A control Group
Experimenter expectancy
Experimenter’s expectation
inadvertently communicated to the
subjects who then produce the desired
effect(Pygmalion Effect)
Solution Double Blind Studies
LEAD TIME BIAS
Gives a false estimates of survival rates
Solution: life expectancy to asses
benefits
RECALL BIAS
Participants inaccurately recall events
in the past
Solution - Confirmation
LATE LOOK BIAS
Individual with severe disease are less
likely to be uncovered in a survey
because they will die first
Solution- class disease in terms of
severity
SAMPLING BIAS
Subjects are not representative of
the population being studied
Solution- random, independent
sample
CONFOUNDING BIAS
The factor that is being investigated is
related to other factors or of less interest
Can obscure a relationship or make it
seems that there is one when there is none
DESIGN BIAS
• Design Bias- parts of the study do not
fit together to answer the question of
interest common issue is no
comparable groups
Solution: Random assignment
PREVENTING BIAS
• Involves the use of :
Blind studies
Placebos
Crossover Studies
Randomized Studies
BLIND STUDIES
• Single blind studies - only the subject does not
know what treatment they are receiving
• Double Blind Studies- neither the subject nor
the evaluator knows what treatment the subject
is receiving or which group received the
treatment or is the control.
• Triple Blind studies – neither the subject, the
evaluator nor the analyst knows what the
treatment the subject and control group
receive.
PLACEBOS
• In a blind study a patient is given a
placebo(in active drug) rather than an
active drug
• Control group receive the placebo while
the experimental group receive the active
drug
• Placebo Effect
CROSSOVER STUDIES
• Two groups
• One group receive the drug the other a
placebo
• Later the placebo group is given the drug and
the group who was initially given the drug is given
the placebo
• Each subject act as their control
RANDOMIZATION
• Subjects are randomly allocated
into the intervention group and
control group
• Regarded as the most
scientifically rigorous study in
epidemiology