Filariasis

Filariasis is a group of parasitic disease caused by a particular class of roundworm. The roundworm is spread by black flies and mosquitoes. The different forms of diseases are characterized by which tissue in which the roundworm settles and the worm is rarely found in other parts of the body or in the blood. 
The parasites have a complicated life cycle that requires injection by the insect vector into two hosts before the worms reach maturity and can start laying their own eggs. When adult worms lay eggs and a host is bitten, the insect transfers the larvae to a new host. As the larvae mature, they must again travel by insect to a new host before they can mature. 
The roundworms are difficult to detect in tests because they are generally only released into the blood to coincide with the feeding habits of the vector insect. As such, blood samples (either from a vein or a finger prick) have to be tested at the right time of day in order to detect the parasite. 
Treatment is with a combination of albendazole and ivermectin. 
Filariasis at Wikipedia