People from traditionally urban areas could be genetically better suited to fighting infection, say researchers.
The University of London team looked at how many people carried a specific gene variant known to give them resistance to TB and leprosy. It was more common in those from areas with a longer history of urbanisation, where the diseases were more likely to have been rife at one point. They described the discovery as an example of "evolution in action".
The phenomenon, reported in the journal Evolution, is suggested as an example of so-called "selective pressure" in relation to disease resistance. It happens because, when a population is exposed to a killer illness, the people who are best placed to pass on their genes to the next generation are those whose genetic make-up helps them fight the infection.
In towns and cities, where people intermingle far more closely, the likelihood of being exposed to infectious disease is theoretically higher. So, over the centuries, the greater the level of historical exposure, the more likely it is that these resistance genes will be spread widely among the population.