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University of Ha'il
Faculty of Public Health
Foundation of Public
Health
Lecture No.: 8
INFECTIOUS
DISEASE
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INFECTIOUS DISEASE
Definitions:

Infectious diseases


Communicable diseases


Transmission – through unnatural routes – from an infected person
Contamination


Transmission – directly or indirectly – from an infected person
Transmissible diseases


Caused by an infectious agent
The mere presence of microbes in or on the body.
Infection

Results when the organism has evaded the body’s external defenses,
multiplied, and become established in the body

Infection


Is the invasion of the host by a pathogen.
Disease

Results only if the invading pathogen alters the normal functions of
the body

Disease is also referred to as morbidity
Manifestations of Disease

Symptoms
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

Subjective characteristics of disease felt only by the patient

Objective manifestations of disease that can be observed or
Signs
measured by others

Syndrome

Group of symptoms and signs that characterize a disease or
abnormal condition

Asymptomatic, or subclinical, infections lack symptoms but may still have
signs of infection
Virulence Factors of Infectious Disease

Pathogenicity

Ability of a microorganism to cause disease.


Virulence



Degree of pathogenicity
Virulence factors contribute to an organisms virulence

Adhesion factors

Biofilms

Extracellular enzymes

Toxins

Antiphagocytic factors
Susceptibility

Ability to become infected with an organism when exposed to it

People may loose susceptibility (or become immune) once they have
been:

infected with the organism

vaccinated
Medications may reduce susceptibility temporarily
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The source and/or reservoir of the pathogen

.
Source:
The location from which the pathogen is immediately transmitted
to the host directly or indirectly

Reservoir :
The site or natural environmental location in which the pathogen
resides and from which infection of the host can occur
Epidemiologic Triad
Disease is the result of forces within a dynamic system
consisting of:

AGENT OF INFECTION

HOST

ENVIRONMENT
Factors Influencing Disease Transmission

AGENT OF INFECTION

Infectivity

Pathogenicity

Virulence

Immunogenicity

Antigenic stability

Survival
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

HOST

Age

Sex

Genotype

Behaviour

Nutritional status

Health status
ENVIRONMENT

Weather

Housing

Geography

Occupational setting

Air quality

Food
1. Infectious Agents

Viruses

Fungi

Protoctists / Protozoa

Helminths
2. 3 conditions required for infection to spread from one person to
another:
1. One person must be infected with a microorganism.
2. The other person must be susceptible to infection with that
microorganism.
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3. The microorganism must be able to leave the body of the infected
person and enter the body of the susceptible person.
3. The infectious Disease cycle
4. Chain of Infection
5. Spread of infection
 Three conditions must be met for infection to spread from person to person
 If any of these conditions are not met, the chain is broken and the infectious
disease does not spread.
6. Spectrum of Disease
 Exposure
 Subclinical manifestations
 Pathological changes
 Symptoms
 Clinical illness
 Time of diagnosis
 Death
 Whether a person passes through all these stages will depend upon infection and
prevention, detection and therapeutic measures
7. Terms Used to Classify Infectious Diseases
8. Portals of Entry
 Sites through which pathogens enter the body
 Four major types
 Skin
 Mucous membranes
 Placenta
 Parenteral route
 Skin
 Outer layer of packed, dead, skin cells usually acts as a barrier to
pathogens
 Some pathogens can enter through openings or cuts
 Others enter by burrowing into or digesting the outer layers of skin
 Mucous Membranes
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 Line the body cavities that are open to the environment
 Provides a moist, warm environment that is hospitable to
pathogens
 Respiratory tract is the most commonly used site of entry – entry is
through the nose, mouth or eyes
 Pathogens able to survive the acidic pH of the stomach may use the
gastrointestinal tract as a route of entry
Mode of transmission
 The way a microorganism is spread from an infected person to a
susceptible person
DIRECT TRANSMISSION
INDIRECT TRANSMISSION
Direct contact
Airborne
Droplet spread
Vehicle borne
Vector borne
 Direct transmission
 The spread of an organism from one person to another as a result
of the direct contact of an infected and a susceptible person
 The modes of direct transmission
 direct contact
 droplet spread
 Direct contact
 The organism is passed to a susceptible person when he or she has
contact with an infected person’s skin, mucus membranes or body
fluid (blood, urine, feces).
 Droplet spread
 The organism is spread to a susceptible person in respiratory
secretions that form when an infected person breaths, coughs or
sneezes.
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 A susceptible person can become infected if the droplet enters his
eyes, nose or mouth directly or via contamination of hands.
 Considered direct transmission because droplets are heavy so they
do not tend to travel more than 6 feet from the infected person
 A susceptible person who is not near an infected person
will not become infected
 The spread of disease by respiratory droplets usually
requires extended contact
 Indirect transmission
 Occurs when an organism is spread from an infected person to a
susceptible person via an intermediary or go-between.
 Via the air = airborne
 Via an insect = vectorborne
 Via an inanimate object = vehicleborne
 Mode of Transmission
 Person-to-person (respiratory, oro-genital, skin)
 Examples: HIV, measles
 Vector (animals, insects)
 Examples: rabies, yellow fever
 Common vehicle (food, water)
 Examples: salmonellosis
 Mechanical vectors (personal effects)
 such as doorknobs, or toothbrushes are called fomites
The course of an infectious disease:
9. 1. The incubation period is the period between pathogen entry
and the expression of signs and symptoms. The pathogen is
spreading but has not reached a sufficient level to cause clinical
manifestation. This period’s length varies with disease
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10. 2. the prodromal stage is the period in which there is an onset of
signs and symptoms, but they are not yet specific enough to
make a diagnosis. The patient often is contagious.
11. 3. The illness period is the phase in which the disease is most
severe and has characteristic signs and symptoms. The immune
response has been triggered; B and T cells are becoming active.
12. 4. in the periods of decline, the signs and symptoms begin to
disappear. The recovery stage often is referred to as
convalescence.
CONTROL OF EPIDEMICS
1. There are three types of control measures.
1. The first type is directed toward reducing or eliminating the
source or reservoir of infection:
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 Quarantine and isolation of cases and/or carriers
 Destruction of an animal reservoir of infection
 Treatment of sewage to reduce water contamination
 Therapy that reduces or eliminates infectivity of the
individual
2. The second type of control measure is designed to break the
connection between the source of the infection and
susceptible individuals.
Examples include :
 sanitation measures:
 Chlorination of water supplies
 Pasteurization of milk and other beverage
 Supervision and inspection of food and food handlers
 Destruction of Vectors
3. The third type of control measure reduces the number of
Susceptible individuals and raises the general level of herd
immunity by immunization.
Example include:
 Passive immunization to give a temporary immunity
following exposure to a pathogen or when a disease
threatens to take an epidemic form
 Active immunization to protect the individual from
the pathogen and the host population from the
epidemic
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 Levels of Disease Occurrence

Sporadic level: occasional cases occurring at irregular intervals

Endemic level: persistent occurrence with a low to moderate level

Hyperendemic level: persistently high level of occurrence

Epidemic or outbreak: occurrence clearly in excess of the expected
level for a given time period

Pandemic: epidemic spread over several countries or continents,
affecting a large number of people
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