Health education. People learning about the cycle of filariasis on a health education programme in India. Filariasis describes a group of tropical diseases caused by various parasitic worms or their larvae, which are transmitted to man by insect bites. Some species of worm cause inflamm- ation of the lymph vessels which can result in elephantiasis (enlargement of the affected area with thickening and cracking of the skin). Other species cause blindness (onchocerciasis), or skin lesions (loiasis). Education about the use of insecticides, nets, and clothing to avoid insect bites can reduce the prevalence of these diseases.