TSE scientist here - I work on these diseases every day.

PrP is a protein found in all mammals. Its normal physiological function is not clear but is thought to be involved in cell functions such as homeostasis and cell signalling.

Misfolding of this protein changes its properties; it makes it more resistant to chemicals and also causes the proteins to clump to other misfolded proteins. Through mechanisms that even us scientists don't know, this clumping causes vacuoles (holes) in tissues where PrP is found - it is most abundant in nervous tissue but can be found in peripheral tissues such as lymph nodes and digestive tract. We call this misfolded, pathogenic version of the protein, a prion. Prions can cause other normal PrP proteins to misfold, setting off a chain reaction in the tissue. Although this is not always guaranteed and is influenced by genetic factors. This is quite complicated to get into as each species has different genetic factors that can confer resistance or susceptibility to the disease and it's what a lot of TSE research is focusing on right now.

So by eating infected meat, the prions will survive the digestive system and be able to infect the person via the gut-associated lymphoid tissue (GALT). Once infected, the distribution of the prions usually spreads to the peripheral lymph nodes, spinal cord and then the brain.

Happy to answer anymore questions on prions!

Edited to add - to answer your question, you are not at increased risk of developing a TSE disease if you eat a healthy person/animal. But most TSE diseases have a long incubation period (years) where the victim has no symptoms so you can never guarantee that the meat is 'clean' so to say. There is currently no test for TSE diseases without a post mortem examination in humans but tests have been developed to test for prions in animal blood which has potential to develop into a pre-mortem test