Terence McKenna talks about the correlation between species that have a strong social life and susceptibility to diseases and viruses.
“Diseases want to be able to move from one person to another. The rise of this susceptibility to disease is something that apparently also was going on at the same time that we were acquiring language, beginning to domesticate animals, switching our diet. What we really are is the evolutionary wreckage of a very chaotic and crazy series of pattern shifts that went on between 50,000 and 100,000 years ago. Now we are the inheritors of that.”