testing for prions, especially for ones that you don't know of yet (which can develop at any time in any person due to the nature of prions, but that's very very rare) is extremely difficult, when mad cow disease is detected in a cattle herd you cull the herd, it isn't worth it, the risk is far too high, and prions cannot be removed through anti-biotics or any kind of treatment, the only way to destroy them is denaturation through burning
prions are misshapen proteins that 'spread' in that when they touch the normally shaped proteins of the same kind they can cause them to become misshapen in the same way too, and being misshapen means this protein can't do its job, and when it's job is keeping your brain working or doing other important biological functions, you can see where issues arise
someone who was healthy at the end of their life may have prions in their body that were dormant and did not show symptoms, the longer you live the more likely you are to have proteins become misshapen, most are harmless and wouldn't spread anyway, and most people die without developing anything to do with prions because of just how unlikely it is to happen
but in the rare chance someone's body produces a misfolded protein that is both infectious and affects an important biological function then that extremely small chance would be buried in the ground or incinerated the majority of the time, but if more people practiced cannibalism then that protein would have a field day as it spread to the first person and then through blood contact spread to others, because a lot of prion disease is slow acting and/or has a very long incubation period (multiple years)
so not practicing cannibalism makes it a lot less likely for an untreatable deadly prion disease to materialize
first of all, the game is alright, and second there isn't actually any sex, it's just a horror game with an extremely abusive manipulative sister/brother dynamic where there are a lot of fucked up things that happen like eating the neighbor, sacrificing souls to a demon, etc, near the end of the game there's a choice you can make that is that the sister gets more serious about making sure her brother is only hers and that implies incest, for some reason people on twitter got really mad at the inclusion of incest as a topic when it's very obviously not an endorsement but a bad theme like all of the others the game gets into