The researchers started from the premise that densely populated cities would be good places for infectious diseases, which could spread easily from person to person. That situation should have set up selection pressure for the ability to survive such infections.

研究人员首先假定，人口密集的城市是传染性疾病的温床，使人际之间的疾病传染更为容易。当然，这一环境必须建立能够抵抗这类传染病并存活下来的选择压力。