Many found it interesting that the prevalence of H. pylori infections did not correlate with cancer incidence. In a study of two Colombian populations, a coastal population of African ancestry had a low incidence of gastric cancer compared to a population of largely Amerindian descent in the Andes Mountains. By studying the tissue samples of patients from these populations, molecular biologists and researchers found out that the H. pylori strain affecting those in the coastal region were of African descent, while the H. pylori strain affecting the Amerindian human population were of south European descent. The results demonstrated that a “shared evolutionary history of humans and bacteria resulted in a less virulent host-pathogen relationship.”

“[It’s] fascinating,” said El-Omar. “If you have African strains affecting African-ancestry hosts, it doesn’t cause too much damage, whereas if you’ve got African-origins strains infecting Amerindians up in the mountains, that’s when you get most precancerous changes. So it looks like if you’ve coevolved with your strains, you get less and less virulence.”

This article didn’t go into too much detail about the evolutionary mechanisms so I did some more research on this host-pathogen coevolution. I found out that one explanation for why one population is more affected by diseases is tolerance evolution. The evolution of tolerance results in a changed selection on parasite populations, which can lead to parasite evolution despite the fact that tolerance is not directly antagonistic to parasite fitness. The evolution of tolerance is like our attempts of vaccination in order to decrease the prevalence of disease, without reducing parasite densities or eradicating the infection. These vaccinations can select for more pathogenic viruses, creating a greater risk for those unvaccinated who come into contact with these pathogens. Tolerant individuals also select for parasites with greater virulence, causing a more devastating effect on those intolerant who become exposed to the disease.

Link to article: http://www.the-scientist.com/?articles.view/articleNo/38845/title/Human-Pathogen-Coevolution/