Disease and the Immune System An infectious or communicable
disease is one that can be transmitted from person to person,
from an infected animal or from the environment to the person.
Illness results when the bacteria, viruses or other agents
invade and multiply in the host (the person's body) and as a
result cause specific diseases. Illness and the development of
resulting disease depend primarily on the bacteria, virus or
other agents and on the reaction of the body's immune system.
Basically, bacteria or viruses can invade the body without
causing the signs and symptoms of disease if the body's immune
system responds adequately. Only when the bacteria or virus
overpower the immune system do the signs and symptoms of disease
develop. When bacteria enter the body, they attack normal cells
by secreting toxins that damage or kill the cells and by
competing with the cells for nutrients. Bacteria are self-
contained, living cells that reproduce rapidly if not checked by
the immune system or appropriate medicatio