This is called “parasite load” and is correlated with most social behaviors around groups.
“The forest region of central Africa is the disease epicentre of the universe! HIV from chimps, bats carrying rabies, Ebola, SARS, insect vectors carrying malaria and parasitic diseases like river blindness and elephantitis, loads of fecal-oral diseases… It’s described as ‘pathogen rain’,” says Walsh.
“You live in a big group with lots of (inter-group) social interaction, and one of you gets Ebola – everybody dies. So it doesn’t make evolutionary sense in such places”.
Walsh suggests that the region’s ‘pathogen rain’ could have stunted early human development, as limited interactions due to fear of disease meant that ideas and innovations were unable to spread and build, leaving our first ancestors languishing in the forest for thousands of years.
“In disease hotbeds, people have much stronger group identification, which makes them much more hostile – part of the behavioural immune system. You see the same in gorillas.”