I gave the OP another example of a deadly and yet widespread virus, HIV. He is not completely off the ball, though. Our immune systems are rather good, so (wit

I gave the OP another example of a deadly and yet widespread virus, HIV. He is not completely off the ball, though. Our immune systems are rather good, so (with significant exceptions like HIV) if some virus doesn't kill us quickly, we tend to clear it out of the organism or at least get it under control, so it won't kill us at all. Viruses that are very deadly but take a long time to kill you (thus enhancing the risk of transmission), like HIV, aren't very typical.