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.Their bites deposit a parasite that travels to the lymph system.Most cases are symptomless. Rarely, long-term damage to the lymph system causes swelling in the legs, arms and genitalia. It also increases the risk of frequent bacterial infections that harden and thicken the skin (elephantiasis).A drug taken yearly can kill the parasites.