When syphilis first appeared in Europe in 1495, it was an acute and extremely unpleasant disease. After only a few years it was less severe than it once was, and it changed over the next 50 years into a milder, chronic disease. The severe early symptoms may have been the result of the disease being introduced into a new host population without any resistance mechanisms, but the change in virulence is most likely to have happened because of selection favouring milder strains of the pathogen. The symptoms of the virulent early disease were both debilitating and obvious to potential sexual partners of the infected, and strains that caused less obvious or painful symptoms would have enjoyed a higher transmission rate.