There are a number of different roundworms, which can cause various diseases. Roundworm infestation occurs throughout the world, affecting large population groups in the tropics and subtropics. Male and female worms exist; some, like the tapeworm, are hermaphrodite. Larvae develop from eggs, generally in the soil (Strongyloi-des, hookworm, eelworm). People become infested by eating contaminated food, by larval penetration of skin, or by eating eggs or larvae in vegetables or meat (including trichomoniasis); wireworms have a mosquito as intermediate host, and the disease is transmitted by its bite. The threadworm is also a roundworm, and can be transmitted by eggs that remain on the hands or by inhaling air in which the eggs are suspended. Good hygiene (sanitary provision, handwashing, etc.) is the best way of preventing infection. Treatment by medication is possible.