Worms are spread between horses by means of the horse's droppings.
Adult worms shed eggs into the horse’s gut, these eggs fall onto the pasture when the horse produces droppings. Another horse can then inadvertently eat the infective eggs when grazing close to the pile of droppings. Once inside the second horse, the eggs hatch and develop into adult worms capable of shedding more eggs.
This is a simplified worm lifecycle. Each species of parasite has its own variation of this basic pattern and there additional complexities to consider. Please follow the below links for the four most common worms of the horse:
Other parasites include: