system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is cannibalism so bad for our health?

One thing that always comes up in any conversation about cannibalism - not that it comes up OFTEN in my circles, I just want to clarify - is that cannibalism is bad for the health of whoever or whatever is partaking. Why is this the case, and why are some species able to indulge in cannibalism on a regular basis with seemingly no adverse effects?
gpt
Diseases often have trouble jumping from one species to another. The closer related to you your food is (evolution wise) the smaller the gap the disease needs to jump. This is why eating bushmeat from apes is more risky than eating some less closely related animals.

The above is true especially for viruses. Parasites are another problem. One thing that has been big in the news are prions.

Prions aren't really living things at all. They are just certain types of proteins which cause the proteins inside of you to become like them by their mere presence. In particular the prions everyone is worried about are ones that are inside your brain and nerves. If you eat another humans brain who has such a prion disease you will become infected yourself.

Normally this is not such a big deal as people aren't in the habit of eating each others brains.

However in cultures where cannibalism was practiced, such disease can become a problem. One example is Kuru also called the laughing diseases which used to be found among cannibals in places like New Guinea.

Another example is mad cow diseases which became a problem when stupid people got into the habit of feeding cows the ground down remains (including brains) of other cows.

In fact mad cow became such a big problem that enough cows with it ended up in the food chain to possibly infect humans despite the species gap. Creutzfel-Jacobs is what the disease is called that many think is what happened if you eat cannibalistic cow brains.

The TL;DR of all this is that if you plan to eat humans make sure to only pick healthy looking ones, stay away from their brains and avoid eating any known cannibalis who seem to have the shakes.