One of the most widespread and most disfiguring diseases - filariasis or elephantiasis - is caused by a thread-like nematode - Wuchereria bancrofti. The disease is usually spread by the mosquito Culex fatigans which, when it bites an infected individual, takes up the filariae worms with the blood meal and injects these into the blood of another victim. Elephantiasis is characterized by infection of the lymph glands which causes swellings throughout the body, mostly of the legs, scrotum and the breasts which sometimes attain dimensions of several feet.
Bancroftian filariasis is spread widely throughout Africa, southern and southeastern Asia, the Pacific and the tropical and subtropical regions of South America. It is common in India and China, where in some areas 50% of the inhabitants are infected, and in the West Indies, South America and Africa. In some of the Pacific Islands, the infection rate is 70%. In 1990 it was estimated that some 90 million people were infected.