Viruses are programs that spread themselves throughout computer systems and networks and can be potentially damaging to a persons computer.
Viruses remain resident in the memory of a computer after they have delivered their payload.
This means that after the virus does its initial damage it remains active in order to continue damaging the computer. A virus can be programmed to attack any part of the computer system. Virus attacks can result in loss of data, damage to hardware or invasion of privacy. A Trojan horse is similar to a virus in that it can cause damage to a persons computer. However, Trojan Horses do not spread themselves to other computers.