Ascaris, a genus of parasitic roundworms. Members of this genus are widely distributed, and are especially numerous in warm, moist climates. Infection with these roundworms is called ascariasis.

The common roundworm—largest of parasitic roundworms—infests humans. The female is 8 to 16 inches (20 to 40 cm) long; the male, 6 to 10 inches (15 to 20 cm). A person usually becomes infested through food or drink that is contaminated with the worm eggs. The larvae hatch out in the host's intestines, enter the blood and lymph, and are carried to the lungs. They then travel through the trachea to the esophagus and stomach, and return to the intestines to mature. Mature females lay fertilized eggs, which are passed out with the feces of the host. Adult worms sometimes emerge from the anus or throat.

Infection with the common roundworm causes any of a variety of medical problems, including nausea, allergic reactions, malnu-trition, anemia, and pneumonia. Treatment for the infection includes drugs that destroy the adult worms. For prevention of infection and of reinfection, sanitary toilet facilities are of great importance.

The common roundworm is Ascaris lumbricoides of the class Nematoda.