(1) Chemical barriers. The host combats the invasion of micro-organisms
by the secretion of chemicals that are antimicrobial in nature. The acid pH of the
stomach, skin, and vagina; the bile salts of the intestines; and the lysozymes of the eyes
and saliva serve as deterents to the invasion of microorganisms.
(2) Physical barriers. The intact skin, mucus (sticky lining of the mucous
membranes), nasal hairs, cilia of the respiratory epithelium, peristaltic movement, and
normal microbial flora (occupying attachment sites) prevent the entrance of
(3) Microbial antagonism. Antiparasitic factors present in the serum and
competition for nutrients from the normal flora also serve as obstacles for the possible
Epidemiology is the science that studies propagation and prevalence of human
disease. It determines the frequency and distribution of a disease in a community.
Epidemiologists, scientists who specialize in epidemiology, are always in search of
specific causes of localized outbreaks of infection. Below are some terms commonly
used in epidemiology.
a. Endemic. A disease is endemic when it is present in a community at all
times but is clinically found in a few individuals and with low morbidity.
b. Hyperendemic. A hyperendemic disease is one that is present in a high
frequency in a community.
c. Sporadic. A low frequency of disease that is not widely diffused is referred to
d. Epidemic. An epidemic occurs when a disease spreads rapidly through a
community and affects a large number of individuals.
e. Mode of Infection. Some parasites can infect the host by direct contact,
while others require a complex mechanism to enter the host.
(1) Congenital. Some parasites can be transmitted to the younger
generation by the older generation. The transmission of malaria, toxoplasmosis and
several other parasitic diseases may be accomplished by parasitic forms crossing the
placental barrier from the mother's blood to the unborn fetus.
(2) Direct contact. Some parasites can complete their life cycle by being
transmitted from an infected site to another site where there is no disease. Then they
can parasitize either the new or the same host.