Lymphatic filariasis is a parasitic disease caused by microscopic, thread-like worms that only live in the human lymph system, which maintains the body's fluid balance and fights infections. It is spread from person to person by mosquitoes. Most infected people are asymptomatic and never develop clinical symptoms. A small percentage of people develop lymphedema, which may affect the legs, arms, breasts, and genitalia; bacterial infections that cause hardening and thickening of the skin, called elephantiasis; hydrocele (swelling of the scrotum) in men; and pulmonary tropical eosinophilia syndrome. Treatment may include a yearly dose of medicine, called diethylcarbamazine (DEC); while this drug does not kill all of the adult worms, it prevents infected people from giving the disease to someone else.