There is some research that suggests that viruses like the flu are really actually kind of at the razor's edge when it comes to mutation. They're mutating so fast that if they mutated much faster they would actually develop a lot of harmful mutations that could slow them down and cripple them and eventually literally drive them extinct.

Everybody can digest milk when they're little.

Borna virus is not a retrovirus. It doesn't actually insert its own genes into our cells. What it does is just hangs out near our DNA and uses some of the molecular machinery to copy itself.

Over millions of years the viruses in our genome mutate more and more so the look less and less and less recognizable as viruses and so if there was a virus that infected our pre mammal ancestors like 250 million years ago, which it probably did, we can't see it because it just looks totally random.

We really depend on viruses for our complete survival.

One of the big challenges now is to figure out just how many viruses there really are in the human genome. So far the estimate is 8.3% of our genome is virus, but it actually could be a lot higher.

The reason that viruses are so hard to fight, the reason for example we need a flu virus every year is that they evolve very fast.