Significant Moments: Influenza

This necessity for viruses quickly to find new bodies to infect explains the fact that the most devastating disease epidemics are often short-lived. Influenza is a prime example. Once a new influenza virus arrives on the scene, it methodically makes its way through the susceptible population, and then, with no one left to infect, abruptly fades away. And were it not for the virus's virtuoso ability to mutate into newly infectious forms, that would be the end of that.

Retroviruses, on the other hand, are simply in no rush. They come on in, make themselves at home, and hang around for a while. In fact they become so much a part of the household that while the infected cell may alert the body that there's a virus around, the immune system's virus fighters simply don't do enough about it. And the virus—that is, the viral genes—taking advantage of such generous laissez-faire, becomes a semipermanent guest. It's a particularly ingenious and efficient way to conduct an infection.
Peter Radetsky, The Invisible Invaders: The Story of the Emerging Age of Viruses.

Funny thing. My mother's father died in the great influenza pandemic in 1918. Coincidentally, my father's father also died of an infectious disease, meningitis.

Wash your hands!