TLDR: The human and primate outer cell membrane is covered by a sugar coating (sialic acids) which protects the cells from foreign microbial invasion. Human cells are coated with one type of sialic acid, N-acetylneuraminic acid (Neu5Ac). But apes and most other mammals also carry a different one, N-glycolylneuraminic acid (Neu5Gc). 2 million years ago a mutation in a certain gene led to overproduction in one type while dicontinuing production of the other type.

This abundance of one type of silaic acid sharpened our immune system to specific microbes but since we discarded the other type or silaic acid, left us also vulnerable to certain microbes.