Computer viruses are programs that can copy themselves and infect computers without permission. True viruses spread when an infected computer is connected to another computer. There are different types of viruses like email viruses, Trojan horses, and worms. Viruses work by executing code and writing to memory when an infected program is launched. Some of the worst viruses in history include Melissa, ILOVEYOU, and MyDoom. People can prevent viruses by using protected operating systems, virus protection software, and avoiding unknown attachments or programs.