Immunity to a parasite develops as a result of exposure to infection with that particular parasite. As a result, young animals that have had less exposure tend to be more susceptible and older animals more resistant to infection. This means that disease and death caused by parasites, especially Strongylus, Parascaris, and Strongyloides, occur more readily in foals. Some older animals that become immune rid themselves of most of the infecting parasites. This is called self-cure. In other cases, parasites are able to evade the host´s immune response and infection persists in the older animals, which results in continuous contamination of the environment. For this reason most older animals require treatment.