Millions of people are infested by parasitic worms, although in Europe parasitic infestations are mainly introduced by tourists and foreign employees. There are three major groups of worms: roundworms, flukes and tapeworms. Males and females occur only in round worms; the other worms are hermaphroditic, having both male and female sex organs. The part of the world where a particular worm occurs depends on the conditions the eggs need in order to develop into larvae. Some require a damp, warm soil in order to be able to develop into a larva which can enter a new host (examples are hookworm and strongy-loids). These worms are often encountered in the tropics. The flagellate worm and the eel-worm are not so demanding; they occur the world over. The commonest worm in Britain is the threadworm. Some larvae can develop only inside another animal species (such as mosquitoes and snails). This species then functions as an intermediate host (vector), while the human is the main host in which the full-grown worms live. This applies for example to threadworms and schistosomes. Humans are infested by the larvae or eggs which either penetrate the skin or are eaten with food. In some types of worms, infection occurs through the bite of a mosquito. Not all infested people fall ill; they become carriers of the worms. The question of whether they fall ill is determined by, among other factors, the number of worms, their size and the resistance of the person infested. Infestation can be detected from the eggs or larvae in the stools. The clinical pictures which arise are very varied and depend on the type of worm. Treatment is usually by medication.