as poliomyelitis, common cold, influenza, measles, mumps, chickenpox, smallpox,
hepatitis, encephalitis, warts, rabies, yellow fever, and lymphogranuloma venereum.
(2) Rickettsiae. These organisms are larger than viruses, but are still very
small intracellular endoparasites. These organisms are transmitted to man by mites,
ticks, fleas or lice, and they produce Rocky Mountain spotted fever, typhus (epidemic
and endemic), scrub typhus (tsutsugamushi fever), Q fever, and Rickettsialpox.
(3) Bacteria. Bacteria are minute, one-celled, organisms that may occur
alone or in large groups called colonies. Significant bacteria can be divided by their
shape into three main groups.
(a) Cocci. Cocci are round, one-celled bacteria. The primary
members of this group are staphylococci, which group themselves in clusters;
streptococci, which arrange themselves in chains; and diplococci, which arrange
themselves in pairs. All are pyogenic (produce pus).
(b) Bacilli. Bacilli are rod-shaped; however, they vary from straight to
irregular-curved and branched shapes. They cause such common diseases as typhoid
fever, diphtheria, tuberculosis, and leprosy.
(c) Spirochetes. Spirochetes are spiral-shaped and can move or twist.
Spirilla and Treponema pallidum are examples. The latter causes syphilis.
(4) Fungi. These extracellular endoparasites or ectoparasites are larger and
higher in the scale of plant life than are the bacteria. They include the yeast and molds,
and produce infections of the skin such as ringworm, and infections of the mucous
membranes such as thrush. Some attack internal organs, especially the lungs and
central nervous system, very often with disastrous results.
(5) Protozoa. These are one-celled animal parasites (either extracellular or
intracellular) that cause such common diseases as malaria and amoebic dysentery.
(6) Metazoa. These many-celled, larger animals include the helminthes
(worms) such as the ascaris, the hookworm, the pinworm, the tapeworms, and the
flukes, as well as the arthropods (mites, lice, and so forth.).
c. Intoxicants. Intoxication is the process of taking any chemical substance
that causes disease or injury into the body. Many substances are very useful in small
amounts, and do not cause intoxication; but the same substances may be very toxic in
larger amounts, and result in severe illness or death.