I too, have considered how it is that a strict vegetarian could become infected with a prion disease. First, vegetarian is given by many to mean one who eats more than vegetables. Interesting, but it is common usage of those who still eat animal products and even fish. It seems that if one ceases eating the carcasses of mammals they are "vegetarians".
The transfer of prion disease can be through more than eating the flesh of an infected animal. Thus, a "vegetarian" places themselves at risk. My concern was with a strict vegetarian. Sadly, they often are placed at risk without knowing it. One who is trying very hard to do right, may take an herb that is in a capsule made of gelatin. This is a high risk material for prion disease. Especially when the source was being imported from the UK. The manufacturers said that it would not transmit prion disease, but they later said they were wrong. The process of making the gelatin did not render the material safe.
We can go on and list a number of ways that a strict vegetarian can be infected, but we have done this in the various topics on CJD, Mad Cow Disease and People, etc. Dental treatment, blood transfusions. surgical instruments, medicines derived from blood and urine, etc. Because a strict vegetarian is infected is not evidence that Alzheimers is not related to prion disease. The evidence suggests that there may be a direct link to prion disease.