By mouth. These abnormal proteins which can lead to brain degeneration are in food containing brain tissue from affected animals. Herds are checked for this so this incidence is very rare. Cooking does not destroy them. There are probably 1-3 cases of this in the us per year.
Ubiquitous. Prion diseases are caused by ingested prions. But normal prions are everywhere throughout the body, a primitive substrate of biology. 1/3 of the wild elk population in the us west appears "infected" with prions, yet are symptom free and apparently edible.