So, here's what I think.

Viruses are perhaps created bits of DNA/RNA sequences used by aliens to cause mutations in life form on other worlds - to cause them to mutate into say, copies of themselves.

However, our human DNA (and perhaps that of animals) evolved to the point where our bodies FIGHT the mutant stuff and prevent a "take over".

Wouldn't that be strange if that's what they were trying to do?

Now WHY they would do something like that I am not quite sure. But who knows what goes through the minds of aliens?

lol