Thus both infectious diseases and populations tend to evolve towards an equilibrium in which the common diseases are non-symptomatic, mild or manageably chronic. When a population that has been relatively isolated is exposed to new diseases, it has no resistance to the new diseases (the population is "immunologically naive"). These people die at a much higher rate, resulting in what is known as a "virgin soil" epidemic. Before the European arrival, the Americas had been isolated from the Eurasian-African landmass. The peoples of the Old World had had thousands of years for their populations to accommodate to their common diseases.