Infectious diseases, a disease
transmissible by infection, are caused by entry into the body of
infectious agents commonly called germs, of which there are four
Plantlike organisms ie. bacteria, yeasts
Organisms of animal origin, principally
protozoa and worms.
The rickettsiae, small organisms that
produce diseases such as typhus fever and Rocky Mountain spotted
Viruses, the causative agents of chicken
pox, influenza, German measles (rubella), mumps, poliomyelitis and
An immunity is developed against many of
these diseases once a person has had them. Thanks to
immunization, a person may also be made immune in many cases without
ever contracting the disease.