From what I know as not being a professional doctor, I would say that it appears to me that viruses get less dangerous over time, not more dangerous. This appears to be because viruses that are overly dangerous kill off their hosts too quickly, limiting their ability to spread, whereas the milder forms don't kill their hosts as easily and spread more broadly, building immunity against the stronger variants over time.

Edit: The result of this, if you look at previous diseases that have plagued humanity (see Black Death), is a high initial death count when the virus breaks out, but the death rate declines as the milder variants spread and people survive them, building immunity to stronger versions, until eventually the virus mostly disappears.