Filariae are nematodes that live as adults in various human tissues. They do not lay eggs, but constantly produce enormous numbers of larvae (microfilariae) in humans. These are found in the skin or blood. Human-to-human transmission occurs via insects: the parasites are thus "arthropod-borne". Animal reservoirs play no role of significance in most places, except in subperiodic Brugia malayi.

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There are several species of filariae, but 6 are commonly pathogenic:

• Wuchereria bancrofti
• Brugia malayi
• Brugia timori
• Loa loa
• Onchocerca volvulus
• Mansonella streptocerca

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There are 2 filariae that are often well tolerated by humans: Mansonella perstans and Mansonella ozzardi. The reason for this tolerance is not known. Dirofilariasis is for the most part only of anecdotal importance and no microfilariae are found in humans. Dracunculiasis is traditionally included among the filariasis, although there are marked clinical differences between this parasite and the other filariae.