Viral and bacterial diseases that kill victims before the illnesses spread to others tend to flare up and then die out. A more resilient disease would establish an equilibrium; if its victims lived beyond infection, the disease would spread further. A similar evolutionary pressure acts upon victim populations, as those lacking genetic resistance to common diseases die and do not leave descendants, whereas those who are resistant procreate and pass resistant genes to their offspring. For example, in the first fifty years of the sixteenth century, an unusually strong strain of syphilis killed a high proportion of infected Europeans within a few months; over time, however, the disease has become much less virulent.