In many cases parasites cause minimal problems to sheep. However, if conditions change to increase worm numbers, disease and even death of the sheep may occur.
Numbers are one.
Sheep generally carry mixed infections and a combination of the above would cause even more severe disease. Sheep have innate and acquired ways of avoiding or overcoming parasite infection. Innate resistance is preexisting resistance. The mechanisms of innate resistance are not understood but may include actions of the sheep immune system. Innate resistance to parasite species Sheep can be infected with one species of parasite but not with others.