Lymphatic filariasis is caused by small parasitic worms in the human lymphatic system, the network of glands and vessels that plays an important role in the maintenance of the fluid balance between the body tissue and the blood. The lymphatic system is also an essential component for the body's immune defence.

These worms are nematodes belonging to the family Filariidae. The adults cause blockages in the lymphatic system, with swelling of the limbs, breasts and genitals. Elephantiasis is the form with painful and disfiguring swelling of the limbs.

According to the WHO, the disease is present in over 80 countries, with 20% or of the world's population, or more than 1 billion people, living at risk of becoming infected.

The disease is mainly transmitted by Southern house mosquitoes (Culex quinquefasciatus) and some species of Anopheles, a.k.a. the malaria mosquito. Aedes polynesiensis is an important vector in the South Pacific.