Question: How long does it take for viruses to become harmless?

Katy Brown answered on 20 Jun 2013:

It depends. The viruses I study are unusual because they become part of the DNA of the animal or person they infect. Every time the animal makes a copy of its DNA, it also makes copies of the virus. However, occasionally it makes mistakes, so the next copy of the virus is slightly different to the last. Depending where this mistake is, the virus can become completely harmless straight away, or can carry on exactly as before. The longer the virus is there, the more mistakes it picks up, and the more likely it is to include a mistake which damages it so much it can’t cause harm any more. So sometimes a virus can be inactivated straight away, and sometimes it takes thousands of years. Lots of the viruses we find are millions of years old, so they have lots and lots of mistakes.
Unfortunately, this only applies to single copies of viruses which have become inheritable. Viruses like HIV produce thousands of copies of themselves, lots of which are full of mistakes, but as long as some of the copies still work the virus can carry on infecting new people.