After about 3 months in the worm's gut, the parasite produces a new spore, triactinomyxon. The fish gets infected upon either eating the worm or can encounter the spore in the water. At the occurrence of infection, the parasite divides rapidly and makes its way to the trout's cartilage. The Whirling disease parasite moves through the skin, muscles, and nerves of the fish to get to the cartilage of the head and spine. The cartilage is actually digested by the parasite as it reproduces.
Brown trout are highly resistant, but often are carriers of the disease.
Brook trout fall somewhere in-between. Whirling disease poses no threat to humans or animals, even if you eat or handle infected fish. Environmental stress in general will cause fish to be more susceptible to the disease. The age of the fish is important as well. In older fish, the cartilage has already ossified and therefore the fish is more resistant and juveniles are most vulnerable.