How do prions disrupt the normal function of an organism and cause diseases such as mad cow disease?

Prions causes abnormal folding of the prion proteins in the brain.

Explanation:

Prions are the abnormal infectious agents composed only of the proteins and no nucleic acids. The prions cause several neurodegenerative diseases in the humans as well as the mammals.

The prions cause the cow mad disease in the cows by affecting the brain of the cow as the prions act on the prion proteins present in the brain only and change their conformation. This leads to the degeneration of the neurons and causes tiny pores in the brain giving sponge-like appearance.

This slows down the mental activity and thus ultimately leads to the death of the cow.