A virus infects executable files; and a worm is an executable file. If the virus reaches a PC already compromised by a worm, the virus will infect the exe files on that PC including the worm when the worm spreads, it will carry the virus with it. Bitdefenders Antimalware Lab identified no less than 40,000 such malware symbioses out of a sample pool of 10 million files. Virus actually infects by mistake another piece of malware and ends up using its capabilities to spread.”]
Source: https://www.bitdefender.com/blog/hotforsecurity/virus-infects-worm-by-mistake/