Collecting water from a stagnant pool. Photo credit: Emily Staub, 2004, The Carter Center.

Today, GWD affects poor communities in remote parts of Africa that do not have safe water to drink. In 1986, there were about 3.5 million GWD cases per year in parts of Africa, Asia, and the Middle East. Thanks to the Guinea Worm Eradication Program, there were fewer than 1,800 cases reported worldwide in 2010. More than 84% of the cases in 2010 were from South Sudan. The remainder of the 2010 GWD cases were from Chad, Ethiopia, Mali, and Ghana, the latter of which has now broken transmission.^[1].

People become infected with Guinea worm by drinking stagnant water containing copepods (tiny "water fleas") that carry Guinea worm larvae (immature forms of the worm). These larvae are eaten by the copepods that live in these stagnant water sources. The larvae need about 2 weeks to mature inside the copepods before they can infect humans. Unsafe stagnant water includes ponds, pools in drying riverbeds, and shallow uncovered wells. Anyone who drinks from contaminated water sources can become infected. GWD is not normally caught from drinking flowing water (rivers and streams)^{[2, 3, 4]}.

GWD transmission has a seasonal pattern. In dry regions, people generally get infected during the rainy season, when stagnant surface water is available. In wet regions, people generally get infected during the dry season, when surface water is drying up and becoming stagnant^{[3, 4]}.

The risk for disease varies by sex, age, profession, and ethnicity. These differences reflect how and where people get their drinking water in different areas and countries. In general, about the same number of men and women get infected. GWD occurs in all age groups but it is more common among young adults 15–45 years old. This may be because of the type of work done by people this age. Farmers, herders, and those fetching drinking water for the household can become infected more often. It is thought they are more likely to drink contaminated stagnant water while away from home. In certain areas, GWD affects some ethnic groups more than others^{[2, 3, 4]}.

The greatest risk for GWD is having GWD the year before. People do not become immune to infection. Many people in affected villages suffer from GWD year after year. This is probably because the same water sources are repeatedly contaminated and conditions that support the spread of disease have not changed. It might also be related to some biological factor of the person that increases susceptibility. Not everyone drinking from the same contaminated water supply will become infected. A few people seem to keep getting infected while others drinking the same water do not^{[2, 4, 5]}.