Filariasis (philariasis) is a parasitic disease and is considered an infectious tropical disease, that is caused by thread-like nematodes (roundworms) belonging to the superfamily Filarioidea, also known as "filariae". These are transmitted from host to host by blood-feeding arthropods, mainly black flies and mosquitoes.

Eight known filarial nematodes use humans as their definitive hosts. These are divided into three groups according to the niche within the body they occupy: 'lymphatic filariasis', 'subcutaneous filariasis', and 'serous cavity filariasis'.

The adult worms, which usually stay in one tissue, release early larvae forms known as microfilariae into the host's bloodstream. These circulating microfilariae can be taken up with a blood meal by the arthropod vector; in the vector they develop into infective larvae that can be transmitted to a new host.

Individuals infected by filarial worms may be described as either "microfilaraemic" or "amicrofilaraemic", depending on whether or not microfilaria can be found in their peripheral blood. Filariasis is diagnosed in microfilaraemic cases primarily through direct observation of microfilaria in the peripheral blood. Occult filariasis is diagnosed in amicrofilaraemic cases based on clinical observations and, in some cases, by finding a circulating antigen in the blood.