Immunity to a parasite may develop 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 due to parasites occur more readily in lambs. Some older animals that become immune rid themselves of most of infecting parasites. This is called self-cure. In other cases, parasites are able to evade the host's immature response, and infection persists in the older animals. For this reason most older animals may require treatment.