communicable diseases, illnesses caused by microorganisms and
transmitted from an infected person or animal to another person
or animal. Some diseases are passed on by direct or indirect
contact with infected persons or with their excretions. Most
diseases are spread through contact or close proximity because
the causative bacteria or viruses are airborne; i.e., they can
be expelled from the nose and mouth of the infected person and
inhaled by anyone in the vicinity. Such diseases include
diphtheria, scarlet fever, measles, mumps, whooping cough,
influenza, and smallpox. Some infectious diseases can be spread
only indirectly, usually through contaminated food or water,
e.g., typhoid, cholera, dysentery. Still other infections are
introduced into the body by animal or insect carriers, e.g.,
rabies, malaria, encephalitis, Rocky Mountain spotted fever. The
human disease carriers, i.e., the healthy persons who may be
immune to the organisms they harbor, are also a source of
transmission. Some infective organisms