University College London researchers have found that in areas with a long history of urban settlements, today's inhabitants are more likely to possess the genetic variant that provides resistance to infection, a university release said.
In ancient cities, poor sanitation and high population densities would have provided an ideal breeding ground for the spread of disease.
Natural selection should mean humans would have developed resistance to disease in longstanding urbanized populations over time.
UCL researchers tested the theory by analyzing DNA samples from 17 different human populations living across Europe, Asia and Africa.
Past exposure to pathogens in urban environments led to disease resistance spreading through populations, with ancestors passing their gene variant resistance to their descendants, they say.
"The results show that the protective variant is found in nearly everyone from the Middle East to India and in parts of Europe where cities have been around for thousands of years" UCL Professor Mark Thomas says.
It's a perfect example of human evolution, another researcher says.
"This seems to be an elegant example of evolution in action," biologist Ian Barns says. "It flags up the importance of a very recent aspect of our evolution as a species, the development of cities as a selective force.
"It could also help to explain some of the differences we observe in disease resistance around the world," Barnes said.