The Infectious Disease Process

by: Franz

· An infection is the result of the invasion of a pathogen, a microorganism that can cause disease, into a host.

· Disease result when the normal tissue function is impaired.

· Transmission of pathogen is accomplished by an infectious source, a vector of spread and a susceptible host.

· Six components of the infectious disease process. Also known as the Chain of Infection.

o Causative agent

o Reservoir

o Portal of exit from reservoir

o Mode of transmission

o Portal of entry into host

o Susceptible host

· Infection control strategies are based on breaking the chain.

1. Causative Agent


    • Bacterium
    • Virus
    • Fungus
    • Parasites
    • Rickettsia
    • Helminthes
    • Prion


    • Pathogenicity – ability to cause disease
    • Virulence – disease severity – invasiveness – ability to enter and move through tissue
    • Infectious dose – number of organism needed to initiate infection
    • Organism specificity – host preference
    • Antigenic variations – viral genetic recombinations
    • Toxigenicity – capacity to produce toxins (poisonous substances)
    • Ability to develop resistance to antimicrobial agents

2. Reservoir

The environment in which the infectious agent lives and multiplies

    • Human – symptomatic case of measles, person with tuberculosis
    • Animal – Rocky Mountain spotted fever from ticks, malaria from mosquitoes, rabies from bats and dogs
    • Environment/Fomites – dust, garden soil, contaminated water

3. Portal of exit from reservoir

Paths by which infectious agent leaves the reservoir

    • Respiratory tract (most common path in human)
    • GI tract
    • Genitourinary (GU) tract
    • Skin – open lesion
    • From bloodstream or tissue by insect bites, hypodemic needles or surgical instrument

4. Mode of transmission

There are four (4) main routes of transmission

a. Contact Transmission

o Direct contact – person to person

o Indirect contact – usually in inanimate object

o Droplet contact – large particles from coughing, sneezing, or talking by an infected person

b. Common Vehicle Route (through contaminated objects)

o Food – salmonellosis

o Water – shigellosis, legionellosis

o Drugs – bacteria resulting from infusion of contaminated infusion product

o Blood – hepatitis B

c. Airborne Transmission

o Droplet nuclei – residue of evaporated droplets that remain suspended in air

o Dust particles in the air containing the infectious agent

d. Vector-borne Transmission

o Mechanical or biological spread from vectors, such as flies, mosquitoes, ticks and rats

5. Portal of entry into host

Paths by which infectious agent enters the human body.

o Respiratory tract

o GI tract

o GU tract

o Direct infection of mucous membrane/break in the skin

a. Parenteral (via blood)

b. Transplacental – from mother to fetus

c. Conjunctiva

6. Susceptible host

a. One who lack effective resistance to infectious agent

b. Factors: influencing susceptibility:

o Number of organism to which host is exposed; duration of exposure

o Age, genetic constitution of host, and general physical, mental and emotional health and nutritional status of host

o Status of hematopoietic system; efficacy of reticuloendothelial system

o Absence of abnormal immunoglobulins

o The number of T lymphocytes and their ability to function



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