How worms cause disease
Worms would not be a problem in horses if they lived in perfect harmony with their hosts. At low levels of infection, parasitic worms are well tolerated and cause no detectable harm. However, a small proportion of horses is very susceptible to worm infection and will develop large burdens. It is these horses that are at risk of suffering from disease due to their parasites.
The diseases caused by heavy parasite infection can be divided into two different categories: