This is a type of computer virus that is responsible for copying itself. This is usually very bad for the computer that it will set itself on. There is a possibility that it will corrupt the system that it is on. There is even a possibility that it will start to delete vital files within the computer.
The reason why it is deadly is it is usually not detected early. It may be inside a large computer program that would not raise your suspicions. There are other types of viruses that can be downloaded on your computer. Having the right antivirus program will greatly help.