The Guinea worm is a parasite that spends part of its life cycle inside copepods (water fleas) and part in a human host. Copepods in contaminated water carry Guinea worm larvae. Drinking contaminated water introduces copepods that carry Guinea worm larvae to the body.
The digestive tract kills the copepods, releasing the larvae. The larvae burrow into the host's abdominal tissue, where they mature and mate. Males die, but over the next several months, females travel to the surface of the body—usually the lower leg or foot.
About a year after infection, the females are now at the surface of the body. The worms release acid that causes blistering and fiery pain, and they burrow their way out, a process that can take as long as 3 months to complete. When the suffering person eases the pain by placing the foot in water, the Guinea worm releases her offspring. These offspring are called first-stage larvae.
Copepods consume the larvae, which now begin the next phase of their life cycle. Inside the copepods, they grow and mature into third-stage larvae. The cycle is complete when humans consume the infected copepods.
Because there is no other animal host in which the worm can complete its life cycle, if we can prevent infection at the source, we can eradicate this parasite. Preventing Guinea worm infection can happen if people avoid exposure by drinking only safe water—water that has been filtered to remove copepods. Another tool to prevent infection is to treat water with pesticides that kill copepods. Breaking the cycle of infection also relies on the compliance of infected individuals, keeping them from immersing blisters in water sources.