CHAPTER 5
Emergence and Global Spread of Infection
An understanding of the principles of epidemiology and the spread of disease is essential
to all medical personnel, whether their work is with the individual patient or with the
community. Most infections must be evaluated in their epidemiologic setting; for
example, what infections, especially viral, are currently prevalent in the community? Has
the patient recently traveled to an area of special disease prevalence? Is there a possibility
of nosocomial infection from recent hospitalization? What is the risk to the patient’s
family, schoolmates, and work or social contacts? The recent recognition of emerging
infectious diseases has heightened appreciation of the importance of epidemiologic
information. Factors that increase the emergence or reemergence of various pathogens
include:
EMERGING INFECTIOUS DISEASES
1. Population movements and the intrusion of humans and domestic animals into
new habitats, particularly tropical forests
2. Deforestation, with development of new farmlands and exposure of farmers and
domestic animals to new arthropods and primary pathogens
3. Irrigation, especially primitive irrigation systems, which fail to control arthropods
and enteric organisms
4. Uncontrolled urbanization, with vector populations breeding in stagnant water
5. Increased long-distance air travel, with contact or transport of arthropod vectors
and primary pathogens
6. Social unrest, civil wars and major natural disasters, leading to famine and
disruption of sanitation systems, immunization programs, etc.
7. Global climate change
8. Microbial evolution, leading to natural selection of multi-resistant agents (e.g.
methicillin-resistant staphylococci, new, highly virulent strains of influenza A
virus). In some instances, these changes can be accelerated considerably by
indiscriminant use of anti-infective agents.
INFECTIOUS DISEASE EPIDEMIOLOGY
I. SOURCES AND COMMUNICABILITY
1. Infectious diseases of humans may be caused by exclusively human pathogens, by
environmental organisms or by organisms that have their primary reservoir in
animals
2. Noncommunicable infections include infections derived from the patient’s normal
flora, infections caused by the ingestion of preformed toxins, and infections
caused by certain organisms found in the environment
3. Noncommunicable infections are not spread from person to person but can occur
as common-source outbreaks
4. Communicable infections require that an organism be able to leave the body in a
form that is directly Endemic = constant presence
5. Epidemic = localized outbreak
6. Pandemic = widespread regional or global epidemic
II. INFECTION AND DISEASE
1. Infection involves multiplication of the organism in or on the host and may not be
apparent
2. Disease represents a clinically apparent injury to the host as a result of infection
3. Inapparent infections are termed subclinical, and the individual is sometimes
referred to as a carrier.
4. Infection can result in little or no illness
5. Carriers can be asymptomatic, but infectious to others
III. INCUBATION PERIOD AND COMMUNICABILITY
1. The incubation period is the time between exposure to the organism and
appearance of the first symptoms of the disease.
2. Incubation periods range from a few days to several months
3. Generally, organisms that multiply rapidly and produce local infections, such as
gonorrhea and influenza, are associated with short incubation periods (eg, 2–4
days).
4. Transmission to others can occur before illness onset
5. The infecting dose of an organism also influences the chance of spread
IV. ROUTES OF TRANSMISSION
1. Horizontal transmission = direct or indirect person-to-person
2. Vertical transmission = mother to fetus
A. Respiratory Spread
1. Infections are transmitted by aerosolization of respiratory secretions with
subsequent inhalation by others
2. Efficiency depends on propulsion of discharges from the mouth, the size of the
aerosol droplets, and the resistance of the infectious agent to inactivation
3. Droplet nuclei are usually less than 6 μm in size
B. Salivary Spread
1. Some infections can be transferred directly by contact with infectious saliva
through kissing.
2. Handwashing is especially important
C. Fecal–Oral Spread
1. Fecal–oral spread involves direct or finger-to-mouth spread, the use of human
feces as a fertilizer, or fecal contamination of food or water.
2. Organisms that are spread in this way must be able to resist the acid in the
stomach, the bile, and the gastric and small-intestinal enzymes
3. Reduced gastric hydrochloric acid can facilitate enteric infections
D. Skin-to-Skin Transfer
1. Skin-to-skin transfer occurs with a variety of infections in which the skin is the
portal of entry
2. An inapparent break in the epithelium is probably involved in infection.
3. Syphilis, ringworm, and impetigo are examples of skin-to-skin transfer
E. Bloodborne Transmission
1. Bloodborne transmission through insect vectors requires a period of
multiplication or within an insect vector
2. Direct transmission through human blood has become important because of blood
transfusions and blood products
3. Parenteral drug abuse is a major risk factor
F. Genital Transmission
1. Spread can occur between sexual partners or from the mother to the infant at
birth.
2. Asymptomatic carriage and recurrence are common
G. Eye-to-Eye Transmission
1. Infections of the conjunctiva may occur in epidemic or endemic form
2. Fomites unsterile ophthalmologic instruments are associated with transmission
H. Zoonotic Transmission
1. Zoonotic = animals to humans
2. Some zoonotic infections are directly contracted from the bite of the infected
animal, while other are transmitted by vectors
I. Vertical Transmission
1. Vertical transmission can occur transplacentally, during birth, or through breast
milk
2. Another form of transmission from mother to infant occurs by contact during birth
with organisms which colonize the cervix or vagina
V. EPIDEMICS
1. Infectivity is the frequency with which an infection is transmitted when there is
contact between the agent and a susceptible individual.
2. Disease index is the number of persons who develop the disease divided by the
total number infected.
3. Incidence and prevalence rates are usually expressed as number of cases per 100,
1000, or 100,000 population
4. Interaction between host and parasite determines extent
5. Attack rates and disease severity can vary widely by age
6. Immune status of a population influences epidemic behavior
7. Immunity in population influences spread
8. Sudden appearance of “new” agents can result in pandemic spread
9. Social and ecological factors determine aspects of epidemic diseases
10. Nosocomial = hospital-acquired
VI. CONTROL OF EPIDEMICS
1. Surveillance is the key to recognition of an epidemic
2. Control measures include food hygiene or arthropod control, isolating infected
individuals and carriers, immunization
GENERAL PRINCIPLES OF IMMUNIZATION
1. Immunization is the most effective method to provide individual and community
protection against many epidemic diseases.
2. Active - stimulation of the body’s immune mechanisms through administration of
a vaccine
3. Live vaccines provide both local and durable humoral immunity
4. Killed or subunit vaccines such as influenza vaccine and tetanus toxoid provide
immunogenicity without infectivity
5. Recent developments have brought greater sophistication to the identification and
purification of specific immunizing antigens and
6. Passive immunization uses preformed antibody obtained from humans, derived
from animals actively immunized to the agent, or produced by hybridoma
techniques
7. Hyperimmune globulins are purified from the blood of subjects with high titers of
antibody to a specific disease
8. Passive immunization has a temporary effect