d. Reproduction. Viral nucleic acids interact with the living host cell to cause it
to form the components of the progeny viruses.
e. Classification. In the past, viruses have been classified by the tissue in
which they produce their ill effects. For example, those acting in nervous tissue were
classified as neurotropic; in glandular tissue as glanulotropic; and in skin as
dermatotropic. Newer classification systems are based on precise biological, chemical,
and physical properties.
f. Relationship to Disease. Viruses produce disease symptoms by actual
destruction of the cells they invade. Some produce toxins which, when absorbed by the
host, add to the syndrome.
g. Common Viral Diseases. A few of the better known diseases caused by
viruses are mumps, chicken pox, measles, influenza, yellow fever, encephalitis, polio,
and parrot fever.
h. General Drug Treatment. In general, viral infections do not respond to
specific therapeutic agents.
Rickettsiae are minute, obligate intracellular parasites, whose natural reservoir is
the arthropods, such as ticks, lice, mites, and fleas. Once thought to be an intermediate
form between viruses and bacteria, they are now known to be a specific category of true
bacteria. Rickettsiae are responsible for Rocky Mountain spotted fever, typhus, trench
fever, and Tsutsugamushi fever. All of these diseases are generally brought under
control with the use of antibiotics.
The following terms are useful in the study of immunizing agents:
a. Immunity--the ability to resist or overcome a specific disease or infection.
(1) Active immunity--immunity attributable to antibody formed as a result of
exposure to an antigen.
(2) Passive immunity--short-term immunity acquired by the injection of
antibodies that have been formed in another individual or species.