commensal:  Describing a relationship between organisms where one organism benefits and the other organism is not harmed by the association.

dracunculiasis:  An infection by the parasitic nematode worm, commonly known as guinea worm.  It is caused when drinking water is contaminated by infected water fleas.

helminths:  Worms that are frequently found as intestinal parasites of humans and other animals.  Included among the helminths are such parasites as the roundworm, whipworm, and hook worm.

hygiene hypothesis:  A hypothesis introduced by the epidemiologist David Strachan that suggests that increasing rates of allergies in industrialized countries have been caused by cleaner living conditions and the reduction of exposure to childhood diseases.  Since the introduction of this hypothesis in 1989, the hygiene hypothesis has expanded to include a number of senarios regarding the correlation between inflammatory diseases and changing living conditions in the industrial world.

schistosomiasis:  An infection caused by the Schistosoma parasite.  Schistosoma exhibits a complex life cycle, first infecting freshwater snails before moving on to vertebrate hosts such as humans.  The infection is common in Asia, Africa, and South America.

selection pressure:  In a biological context selection pressure refers to the environmental and competitive factors that influence the kinds of traits that best help a species survive.

survivor’s bias:  In a population study survivor’s bias refers to the fact that individuals who have died may not be included in the results of the study.  As a consequence, the population in a location with a high mortality rate may seem to be healthier than other populations where the mortality rate is lower.

trachoma:  An intracellular bacterial infection in humans caused by Chlamydia trachomatis.  The infection is common in undeveloped countries, where it is the leading cause of blindness.