branes, may allow pathogens to enter the body. Thus
it is essential that all healthcare workers follow aseptic
technique for such procedures.
The portal of exit (see Fig. 22–1) is the way
the pathogen leaves the body or is shed from the
host. Skin lesions, such as those of chickenpox, con-
tain pathogens that may be transmitted to others by
cutaneous contact. Intestinal pathogens such as the
hepatitis A virus and the cholera bacteria are excreted
in the host’s feces, which may contaminate food
or water and be ingested by another host (this is called
the fecal–oral route of transmission). Respiratory
pathogens such as influenza and measles viruses
are shed in respiratory droplets from the mouth and
nose and may be inhaled by another person. The
pathogens of the reproductive tract, such as the bacte-
ria that cause syphilis and gonorrhea, are transmitted
to others by sexual contact. Notice that with respect to
epidemiology, the pathogen travels from one host’s
portal of exit to another host’s portal of entry.
RESERVOIRS OF INFECTION
Some pathogens cause disease only in people. Measles,
whooping cough, syphilis, and bacterial meningitis are
strictly human diseases. To acquire such a disease, a
person must be exposed to someone who has the
illness.
Also of importance is that upon recovery from
some diseases, the host may continue to harbor the
pathogen and thus be a reservoir of it for others. Such
a person is called a carrier. Diseases for which the car-
rier state is possible include typhoid, diphtheria, and
hepatitis B.
Many other diseases, however, are really animal
diseases that people acquire in certain circumstances.
These diseases are called zoonoses(singular: zoono-
sis) and include plague, Lyme disease, encephalitis,
and Rocky Mountain spotted fever, which are spread
from animal to person by vectors such as ticks or fleas.
Rabies is acquired by contact with infected animal
saliva or infected tissue, the virus entering the new
host through breaks in the skin. Salmonellosis is a type
of food poisoning caused by the intestinal bacilli of
animals that contaminate meats such as chicken and
turkey. Prevention of such diseases depends upon
knowledge of how they are spread. For example, peo-
ple who live in areas where Lyme disease is endemic
should be aware that the disease is acquired by way of
a tick bite. If children and pets are examined for ticks
after they have been out of doors, the chance of
acquiring Lyme disease is greatly diminished.
Some bacteria are pathogenic only by accident, for
their natural habitat is soil or water, where they act as
decomposers. The bacteria that cause gas gangrene,
tetanus, and botulism are normal soil flora and cause
disease when they (or their toxins) contaminate a skin
wound, or, in the case of botulism, the toxin is present
in food.SPREAD OF INFECTION
Based on our knowledge thus far, we can classify infec-
tious diseases as noncommunicable or communicable.
A noncommunicabledisease is one in which a resi-
dent species causes disease under certain conditions or
in which a non-resident species causes disease when it
enters the body. Such diseases cannot be transmitted
directly or indirectly from host to host. Cystitis caused
by E. coliin a hospital patient, for example, is not com-
municable to the nurses who care for that patient.
Similarly, a nurse caring for a patient with tetanus or
botulism need not worry about acquiring these dis-
eases; both are noncommunicable.
A communicable disease is one in which the
pathogen may be transmitted directly or indirectly
from host to host. Direct spread of infection is by way
of cutaneous contact (including sexual contact), respi-
ratory droplets, contaminated blood, or placental
transmission from mother to fetus. Indirect spread
is by way of contaminated food or water, or vectors,
or fomites, which are inanimate objects that carry
the pathogen. Influenza and cold viruses, for example,
can survive outside their hosts for a time, so that
objects such as eating utensils, door knobs, or chil-
dren’s toys may be vehicles of transmission for these
pathogens.
Some communicable diseases may also be called
contagious, which means that they are easily spread
from person to person by casual cutaneous contact or
by respiratory droplets. Chickenpox, measles, and
influenza are contagious diseases. In contrast, AIDS is
not contagious, because sexual contact, blood contact,
or placental transmission is necessary to acquire the
virus (HIV). HIV is not spread by cutaneous contact
or by respiratory droplets.METHODS OF CONTROL
OF MICROBESMicroorganisms are everywhere in our environment,
and although we need not always be aware of their
presence, there are times when we must try to dimin-504 An Introduction to Microbiology and Human Disease