A communicable disease is one in which the causative organism or pathogen
is carried from one person to another either directly or indirectly. Communicable
diseases are also called as infectious diseases. These diseases are caused by
microorganisms and are 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 water or food, e.g.,
typhoid, cholera and dysentery. Still other infections are introduced into the body
by animal or insect carriers, e.g., rabies, malaria, encephalitis, rocky mountain
spotted fever etc.. 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 require specific circumstances for their transmission, i.e.,
sexual contact in syphilis and gonorrhea, injury in the presence of infected soil or
dirt in tetanus, infected transfusion blood or medical instruments in serum hepatitis
and sometimes in malaria. In the case of AIDS, while a number of different
circumstances will transmit the disease, each requires the introduction of a
contaminant into the bloodstream.


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