Figure 1-1. The cycle (chain) of infection.
The causative agent is the organism, which produces the disease. Disease-
producing organisms include viruses, rickettsiae, protozoa, bacteria, fungi, and parasitic
a. Viruses. Viruses are extremely small infectious agents. They are so small
that they cannot be seen with a regular microscope. Viruses live inside the cells of the
body. When a virus is outside a living cell, it loses its ability to grow and reproduce. It
remains in this dormant state until it comes into contact with another living cell which it
can invade. An example of a disease caused by viruses is influenza (the "flu").
Different types of influenza viruses produce different types of flu. The common cold,
measles, and mumps are also the result of viral infection.
b. Rickettsiae. Rickettsiae are larger than viruses. They can be seen with a
microscope, but are smaller than most other microscopic organisms such as bacteria.
Like viruses, rickettsiae cannot reproduce unless they are inside a living cell. They are
commonly found in the digestive system of lice, fleas, ticks, and mites. Examples of
diseases caused by rickettsiae include typhus and spotted fever.
c. Protozoa. Protozoa are one-celled animals. They are the simplest
organisms in the animal kingdom. They are much larger than rickettsiae, but you still
need a microscope to see them. Diseases caused by protozoa include malaria and
d. Bacteria. Bacteria are one-celled plants. Diseases produced by bacteria
include streptococcal sore throat (strep throat) and pneumonia.