Filaria, parasitic roundworm (class Nematoda) that can live in the bodies of human beings or animals. The larvae (young worms) are born alive, and can be seen in the blood near the skin of their host (animal or human in which they live). When an insect bites the host, it takes the filaria with it, then releases the filaria into whatever host it bites next. In this way, the filaria continues to infect new hosts. Commonly found in tropical and subtropical countries, the filaria can cause inflammation and disease in the animal and human tissues in which it settles.