How does a virus invade your body? With the help of medical animator David Bolinsky, Robert Krulwich explains how a flu virus can multiply with the help of a single cell in this NPR video from 2009.

There’s also more to read at Krulwich Wonders, including this:

In our video we ask, if a flu virus inside your body can multiply by the millions within seconds, why don’t we topple over and die quickly?

Here’s a better, longer answer than the one in the video. First, some new viruses get caught in mucus and other fluids inside your body and are destroyed. Other viruses get expelled in coughs and sneezes. Second, lots of those new viruses are lemons. They don’t work that well. Some don’t have the right “keys” to invade healthy cells so they can’t spread the infection. And third, as the animation shows, your immune system is busy attacking the viruses whenever and wherever possible.

That is why most of the time, after a struggle (when you get a fever and need to lie down), your immune system rebounds, and, in time, so do you.

Another NPR favorite: Catching Up with the Flu. Or watch more videos about how bodies work.