About two million years ago, humans experienced a genetic change that differentiated us from most primates. This change protected us from some diseases, but caused current consumer products, such as red meat, to pose a high risk to health.
At that time in evolution, a gene called CMAH - which allows the synthesis of a sugar called Neu5Gc - was deactivated. This carbohydrate is found in red meat, some fish and dairy products. If humans consume products derived from animals that have the gene, the body suffers an immune reaction to sugar, which is a foreign substance in the body. This can cause inflammation, arthritis and even cancer.
Happened to find this today while following another link. It frustrates me because my science education is non existent, it does underscore how complicated everything is and I have no way to verify if there is any validity to this