Lesson 1: Introduction to Disease Transmission and Epidemiology

1-6

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1-6. DISEASE AGENTS

A disease-producing agent is any living organism or toxic substance (frequently produced by an organism) that may cause death, disease, or infection in another living organism.

a. Disease-producing infectious agents may be either chemical agents or infectious agents

(1) Chemical agents causing disease may be toxic metals or their oxides (lead, zinc, and so forth); poisonous chemicals such as pesticides (DDT, malathion, and so forth); or toxins produced by living organisms as a result of their metabolic processes (Clostridium botulinum, Staphylococcus aureus, and so forth.

 

(2) Disease agents may also be living organisms, such as viruses, rickettsia, bacteria, fungi, protozoa, and helminths (parasitic worms). These agents will be discussed in detail in Lesson 2.

b. There are several factors that affect the agent’s ability to cause an infection:

(1) Pathogenicity--the ability to produce disease. This factor varies widely between the various categories of agents.

 

(2) Virulence--the agent's ability to overcome the resistance of the host.

 

(3) Resistance--the resistance of the agent itself to drying, disinfectants, and therapeutic drugs.

 

(4) Infectivity--the ability of the agent to penetrate, multiply, and produce change in the host.

 

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