One hundred and twenty million people in 83 countries of the world are infected with lymphatic filarial parasites, and it is estimated that over 1.3 billion (20% of the world's population) are at risk of acquiring infection. Ninety percent of these infections are caused by Wuchereria bancrofti, and most of the remainder by Brugia malayi. For W. bancrofti, humans are the exclusive host, and even though certain strains of B. malayi can also infect some feline and monkey species, the life-cycles in humans and in these other animals generally remain epidemiologically distinct, so that little overlap exists. The major vectors for W. bancrofti are culicine mosquitoes in most urban and semi-urban areas, anophelines in the more rural areas of Africa and elsewhere, and Aedes species in many of the endemic Pacific islands. For the Brugian parasites Mansonia species serve as the major vector, but in some areas anopheline mosquitoes are responsible for transmitting the infection. Brugian parasites are confined to areas of east and south Asia, especially India, Malaysia, Indonesia, the Philippines and China.