A computer virus is a program written to alter the way a computer operates, without permission or knowledge of the user. It must execute itself. It will often place its own code into another program.

The virus attaches itself to a program or file enabling it to spread from one computer to another, leaving infections as it travels. Almost all viruses are attached to an executable file, which means the virus may exist on your computer but it actually cannot infect your computer unless you run or open the malicious program.

A computer worm is a program that spreads quickly and automatically over a computer network. A worm doesn't alter or delete files but instead resides in computer memory, eating up system resources and slowing down the computer. They do not attach themselves to other files or programs. Worms send themselves from one computer to another through the network.

A Trojan horse is a type of virus that, when activated by downloading or opening the file, accesses vital systems to cause damage to a computer.

A virus hoax is a false warning about a computer virus.

Note: Always keep your computer and anti-virus software update to date. Go to the Viruses & Other Malware page for information on what to do to prevent or remove viruses.