https://en.wikipedia.org/wiki/Welchia
Welchia, also known as the "Nachi worm", is a computer worm that exploits a vulnerability in the Microsoft remote procedure call (RPC) service similar to the Blaster worm. However, unlike Blaster, it first searches for and deletes Blaster if it exists, then tries to download and install security patches from Microsoft that would prevent further infection by Blaster, so it is classified as a helpful worm. Welchia was successful in deleting Blaster, but Microsoft claimed that it was not always successful in applying their security patch.[
The viral genes can go dormant in your genome until some future event, often significant stress. That's how chicken poxs become shingles 50+ years later.