however, these cases are very important, because persons with subclinical or
asymptomatic diseases can frequently transmit the diseases as efficiently as those
persons manifesting all the symptoms.
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
a. Disease-producing infectious agents may be either chemical agents or
(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.
Virulence--the agent's ability to overcome the resistance of the host.
(4) Infectivity--the ability of the agent to penetrate, multiply, and produce
change in the host.
SOURCES OF DISEASE (RESERVOIRS)
The source of disease may be a case, a carrier, or an animal.
a. Case. A person who is actually ill with a disease is called a case. A case is a
common source of infection.
b. Carrier. A person who harbors disease organisms but who is not ill is called
a carrier. This person can spread the germs in the same manner as the case. Actually,
he is more dangerous because he may not know that he is harboring the infectious
c. Animal. An animal can actually be ill with disease or it can harbor the
organisms, spreading it to humans in either instance. The term animal means any
member of the animal kingdom, thus including insects as well as mammals.