Encyclopedia of Health and Medicine

P–R
parasite An organism that requires coexistence
with another organism for its survival. The parasite
typically draws nourishment and other needs
from its host organism without contributing in
return to the host’s survival. Some parasites can
survive away from their hosts for limited periods
of time or defined portions of their life cycles.
Some parasites are host-specific whereas others
can adapt to various hosts.
Pathogenic parasites are those that cause INFEC-
TION and disease. Common pathogenic parasites
include flukes, worms, and PROTOZOA (amebas).
They may infect the SKIN or migrate to internal
organs such as the LUNGS, LIVER, or BRAIN, where
they often form cysts. Treatment for parasitic
infections and illnesses depends on the parasite
and the illness.
People who travel to tropical regions or areas
where community sanitation is substandard may
acquire parasitic infections otherwise uncommon
in their home regions. Many systemic parasitic
infections cause gastrointestinal symptoms such as
DIARRHEA. These infections are usually contagious,
spread through fecal–oral contact (contact with
surfaces and substances such as food or water that
are contaminated with particles of feces). Diligent
PERSONAL HYGIENE, especially HAND WASHING, and
appropriate FOOD SAFETY practices are key preventive
measures.
AMEBIASIS
CRYPTOSPORIDIOSIS
GIARDIASIS
microsporidiosis
SCABIES
COMMON PARASITIC INFECTIONS
BABESIOSIS
CYCLOSPORIASIS
MALARIA
PEDICULOSIS
TRICHOMONIASIS
See also BACTERIA; FUNGI; MICROBE; VIRUS.
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pathogen A MICROBE capable of causing illness.
The most common pathogens are BACTERIA, fungi,
parasites, and viruses. The process through which
a pathogen, also called an infectious agent, causes
illness is pathogenesis. The body attempts to protect
itself from pathogens through numerous
mechanisms, key among them being ANTIBODY-
MEDIATED IMMUNITY and CELL-MEDIATED IMMUNITY.
Vaccines and treatments with ANTIBIOTIC MEDICA-
TIONS, ANTIVIRAL MEDICATIONS, and ANTIFUNGAL MED-
ICATIONS
are among the methods available to
contain and eradicate pathogens once they establish
infection in the body.
See also FUNGUS; PARASITE; PROTOZOA; VACCINE;
VIRUS.
pertussis An illness resulting from INFECTION with
the VIRUS Bordetella pertussis. Pertussis is among the
childhood diseases for which routine IMMUNIZATION
is the standard of care in the United States. The
hallmark of the illness is a rapid, violent cough
that causes the person to make a “whooping”
sound when trying to breathe through the coughing,
hence the common term whooping cough.
The cough can be severe enough to prevent
BREATHING. Pertussis was once a leading cause of
death among children under age 5. Though
immunization has dramatically reduced infection,
pertussis may still be fatal in very young children
and very old adults. IMMUNITY, either natural (following
infection and illness) or via VACCINE, lasts
about 12 years.
The unmistakable cough is the primary symptom
and begins about seven days after exposure. In
untreated pertussis, the cough worsens rapidly and
may continue for as long as eight weeks. Many people
also experience VOMITING with the coughing.
The doctor often makes the diagnosis on the basis