is not really practical, but if the human population were sufficiently cleared of parasites by treatment with filaricides then one year without any human reservoirs of infection should suffice to exterminate the parasite. That is the intention of for example, the Global Programme to Eliminate Lymphatic Filariasis, which aims to interrupt transmission in that way. Life cycle of Filarioidea The mature worms live in the body fluids and cavities of the definitive hosts, or predominantly in particular tissues. Details vary according to species. Some of the worst pathogens invade lymphatic vessels and may be numerous enough to clog them. Some species invade deep connective tissues; some infest subcutaneous connective tissue, causing unbearable itching. Some invade the lungs or serous cavities such as the pleural cavity, or pericardial cavity. Wherever established, they may survive for years, the fertilized females continuously producing motile embryos called microfilariae rather than eggs. A microfilaria cannot reproduce in the definitive host and cannot infect another definitive host directly, but must make its way through the host's body to where an intermediate host that acts as a vector can swallow it while itself acting as an ectoparasite to the definitive host. It must succeed in invading its vector organism fairly soon, because, unlike adult filarial worms, microfilariae only survive for a few months to a year or two depending on the species and they develop no further unless they are ingested by a suitable blood-feeding female insect. In the intermediate host the microfilaria can develop further till the vector conveys it to another definitive host. In the new definitive host the microfilaria complete the final stage of development into sexual maturity; the process takes a few months to a year or more depending on species. The mature filaria then must mate before a female can produce the next generation of microfilariae, so that invasion by a single worm cannot produce an infection. Accordingly, it takes years of exposure to infections before a serious disease condition can develop in the human host. Once a new generation of microfilariae is released in the primary host, those in turn must seek out host tissue suited to the nature of the vector species. For example, if the vector is a skin-piercing fly such as a mosquito the microfilaria must enter the peripheral blood circulation, whereas species borne by skin-rasping flies such as Simuliidae and skin-cutting flies such as Tabanidae tend to