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.

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.

It used to be thought that only a certain kind of virus could get into our genome and it's called a retrovirus and that's a virus that might be HIV for example.

We really depend on viruses for our complete survival.

Evolution has led to some populations of people being able to digest milk without much trouble when they're adults as well.

We have Borna virus genes. We're part Borna virus, which is weird, but apparently our cells and our genomes in a weird way might actually be grabbing these viruses, grabbing genetic material from the viruses that are infecting it and pulling them into their own genome.

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.