An anonymous reader writes "Bitdefender reports that there exist viruses which, when they encounter other viruses, will merge and combine effects so that they create a new virus. '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. Although this happens unintentionally, the combined features from both pieces of malware will inflict a lot more damage than the creators of either piece of malware intended. While most file infectors have inbuilt spreading mechanisms, just like Trojans and worms (spreading routines for RDP, USB, P2P, chat applications, or social networks), some cannot replicate or spread between computers. And it seems a great idea to “outsource” the transportation mechanism to a different piece of malware (i.e. by piggybacking a worm).'"