Infectious Disease
According to WHO:
Infectious diseases are
caused
by
pathogenic
microorganisms, such as
bacteria, viruses, parasites
or fungi; the diseases can
be spread, directly or
indirectly, from one person
to another.
Zoonotic
diseases
are
infectious
diseases
of
animals that can cause
disease when to transmitted
humans.
Difference between infectious
disease and others
 Other diseases
 Deals with one population
 Risk  case
 Identifies causes
Infectious disease
 Two or more populations
 A case is a risk factor
 The cause often known
infectious diseases
 Humans:
 Infectious agents:
Helminths, bacteria, fungi, protozoa, viruses.
 Vectors:
Mosquito (protozoa-malaria),
Blackfly (microfilaria-onchocerciasis)
 Animals:
Dogs and sheep/goats – Echinococcus
Mice and ticks – Borrelia
infectious disease
A case is a risk factor …
 Infection in one person can be transmitted to others
Routes of transmission
Direct
Indirect
 Skin-skin
 Herpes type 1
 Across placenta
 toxoplasmosis
 Through breast milk
 HIV
 Sneeze-cough
 Influenza
 Food-borne
 Salmonella
 Water-borne
 Hepatitis A
 Vector-borne
 Malaria
 Air-borne
 Chickenpox
Exposure:
A relevant contact – depends on the agent
Skin, sexual intercourse, water contact, etc.
Factors Influencing
Disease Transmission
Agent
Environment
• Weather
• Infectivity
• Housing
• Pathogenicity
• Geography
• Virulence
• Occupational setting
• Immunogenicity
• Air quality
• Antigenic stability
• Food
• Survival
• Age
• Sex
Host
• Genotype
• Behaviour
• Nutritional status
• Health status
Infectious diseases
according to its
dangerous effects:
1)
2)
3)
4)
Tuberculosis
Malaria
AIDS and
Neglected Diseases
Understanding
infectious disease
Infectious diseases are responsible for the million of deaths each
year in developing countries.
More than 9 millions new cases of and 0.5 million new cases of
drug resistance TB occur each year.
Each year, there are 1.3 to 3 million malaria deaths, up to 5
million of cases of malaria reported in Africa.
World Health Organization, December 2006. An estimated 39.5
million people are living with HIV.
CONTI…
Important increases in Eastern Europe and Central Asia,
where there are some indications that infection rates have
risen by more than 50% since 2004.
In 2006, 2.9 million people died of AIDS-related illnesses.
Young people (under 25 years old) account for half of all new
HIV infections worldwide – around 6,000 become infected
with HIV every day.
There were 4.3 million new infections in 2006.
INFECTION
It is an ability of organism to enter, survive and multiply in
host to initiate certain symptoms.
TYPES OF INFECTION
Bacterial Infection
Viral Infection
Fungal Infection
Parasitic Infestation
Page 10
INFECTIOUS DISEASE
In medicine Infectious disease or Communicable disease is a
disease caused by biological agents such as bacteria, virus or
parasite. Infectious disease also known as Contagious disease or
Trasmissible disease and also includes Communicable disease
comprise clinically evident illness.
Page 11
Infectious Diseases - Definitions
• Disease – a pathological condition of body parts
or tissues characterized by an identifiable group of
signs and symptoms.
• Infectious disease – disease caused by an
infectious agent such as a bacterium, virus,
protozoan, or fungus that can be passed on to
others.
• Infection – occurs when an infectious agent
enters the body and begins to reproduce; may or
may not lead to disease.
• Pathogen – an infectious agent that causes
disease.
• Host – an organism infected by another organism.
• Virulence – the relative ability of an agent to
cause rapid and severe disease in a host.
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
PATHOGENS (patho:disease,gen:produce)
An agent of disease,a disease producer. A term
pathogen most commonly used to refer to
infectious organism.
It basically includes:
Bacteria (eg.staph)
Virus (eg.yeast)
Protozoa (eg.trophozoites)
Multicellular parasites
Aberrant proteins known as Prions
Page 13
CLASSIFICATION
Primary pathogens:
Cause disease as a result of their presence or activity within the
normal, healthy host, and their intrinsic virulence is, in part, a
necessary consequence of their need to reproduce and spread.
Opportunistic pathogens:
Opportunistic disease may be caused by microbes that are
ordinarily in contact with the host, such as pathogenic bacteria or
fungi in GI tract and upper respiratory tract. They may also result
from microbes acquired from other host or environment.
Page 14



are caused by microorganisms or other
agents, such as bacteria, fungi, or viruses.
enter the body of an organism, proliferate,
and cause infection.
can be spread, directly or indirectly, from
one person to another.
Infectious Disease Agents
• Most infectious agents that cause
disease are microscopic in size and thus,
are called microbes or microorganisms.
• Different groups of agents that cause
disease are:
– Bacteria
– Viruses
– Protozoa (Protists)
– Fungi
– Helminths (Animals)
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
How Infectious Agents
Cause Disease
• Production of poisons,
such as toxins and
enzymes, that destroy
cells and tissues.
• Direct invasion and
destruction of host cells.
• Triggering responses
from the host’s immune
system leading to disease
Robin Cochran-Dirksen (adapted
signs and symptoms.
from BioEdOnline Baylor Christine
Courtesy of CDC
Human
Immunodeficiency Virus.
HIV-1 virions can be seen
on surface of
lymphocytes.
Causes of Infectious
Diseases:
Bacteria: These organisms are
responsible for illnesses such as
strep throat, urinary tract
infections, and tuberculosis.
Viruses: Smaller than bacteria,
viruses cause a multitude of
diseases ranging from the
common cold to AIDS.
Fungi. Many skin diseases, such
as ringworm and athlete's foot, are
caused by fungi. Other types of
fungi can infect your lungs or
nervous system.
Parasites. Malaria is caused by a
tiny parasite that is transmitted by
a mosquito bite. Other parasites
may be transmitted to humans
from animal feces.
Table 1. Top five pathogens contributing to domestically
acquired foodborne illnesses
Pathogen
Estimated number of illnesses
Norovirus
5,461,731
Salmonella, nontyphoidal
1,027,561
Clostridium perfringens
965,958
Campylobacter spp.
845,024
Staphylococcus aureus
241,148
Phases of Infectious Disease
• Incubation period – time between infection and
the appearance of signs and symptoms.
• Prodromal phase – mild, nonspecific symptoms
that signal onset of some diseases.
• Clinical phase – a person experiences typical
signs and symptoms of disease.
• Decline phase - subsidence of symptoms.
• Recovery phase – symptoms have
disappeared, tissues heal, and the body regains
strength.
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
Classification of Infectious Disease
• By duration
– Acute – develops and runs its course quickly.
– Chronic – develops more slowly and is usually less severe, but
may persist for a long, indefinite period of time.
– Latent – characterized by periods of no symptoms between
outbreaks of illness.
• By location
– Local – confined to a specific area of the body.
– Systemic – a generalized illness that infects most of the body
with pathogens distributed widely in tissues.
• By timing
– Primary – initial infection in a previously healthy person.
– Secondary – infection that occurs in a person weakened by a
primary infection.
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
Transmission of Infectious
Diseases
• Agents that cause infectious diseases
can be transmitted in many ways.
– Through the air
– Through contaminated food or water
– Through body fluids
– By direct contact with contaminated
objects
– By animal vectors such as insects,
birds, bats, etc.
Courtesy of VOA
Chinese students
wearing masks during a
SARS outbreak
Courtesy of CDC
Aedes aegypti mosquito
Known to transmit
Dengue fever
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
Related Terms
• Endemic/Enzootic: The constant presence of a
disease or infectious agent within a given
geographic area.
• Epidemic/Epizootic: The occurrence in an area
of a disease or illness in excess of what may be
expected on the basis of past experience for a
given population (in the case of a new disease,
such as AIDS, any occurrence may be considered
"epidemic").
• Pandemic/Panzootic: A worldwide epidemic
affecting an exceptionally high proportion of the
global population.
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
Example of an Infectious Disease
- AIDS
• AIDS (Acquired Immune Deficiency
Syndrome) is the disease caused by
the virus called HIV (human
immunodeficiency virus).
• HIV attacks cells of the immune system
and destroys their ability to fight
infection by other agents.
• HIV is spread through the direct
exchange of body fluids.
• There is a long period of time from HIV
infection to the onset of AIDS.
• Anti-HIV drugs prolong the length and
quality of life, but there is no vaccine or
cure for AIDS.
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine
Courtesy of CDC
This highly magnified
transmission electron
micrographic (TEM)
image revealed the
presence of mature forms
of the human virus
immunodeficiency (HIV)
in a tissue sample under
investigation.
Reducing the Spread of
Infectious Diseases
• Vaccines
• Antimicrobial drugs
• Good personal hygiene
and sanitation
• Protection against mosquitoes
• Quarantine
Robin Cochran-Dirksen (adapted
from BioEdOnline Baylor Christine