Viruses, Worms, and Trojan horses Viruses are computer programs that can damage the data or software on your computer or steal the information stored on your computer. These viruses can reach your computer, without your knowledge, through the Internet or through storage devices, such as floppy disks and CD-ROMs. Worms are viruses that replicate themselves once they attack a computer, making it difficult to remove them. A Trojan horse is also a kind of virus disguised as useful software, such as a game or a utility. Once a Trojan horse reaches your computer, it starts acting like a virus causing damage to the computer's data.