Roundworms are common parasites that have a direct life cycle, which means they don't use intermediate hosts. Human roundworm (Ascaris lumbricoides) infections are common in areas with poor sanitation, such as rural regions of the southeastern United States (US).
Female roundworms lay eggs that pass into the environment with the host's feces. Fertilized eggs become infective in about 3 weeks.
An ingested, fertilized roundworm egg hatches in its host's small intestine and penetrates the intestinal wall. Roundworm larvae molt twice before becoming an adult.
Some roundworm larvae migrate to the lungs where they mature for 10 days to 2 weeks. The larvae then ascend into the throat where the host swallows them after induced coughing.
Roundworms reach adulthood and start mating in about 2 to 3 months. Roundworms can live up to 2 years.
Female roundworms produce about 240,000 eggs every day. Roundworms were discovered in Peruvian human remains dating back to 2277 B.C.