Malicious computer programs that are often sent as an email attachment or a download with the intent of infecting your computer, as well as the computers of everyone in your contact list. Just visiting a site can start an automatic download of a virus.
What they can do:
A) Send spam.
B) Provide criminals with access to your computer and contact lists.
C) Scan and find personal information like passwords on your computer.
D) Hijack your web browser.
E) Disable your security settings.
F) Display unwanted ads.
When a program is running, the virus attached to it could infiltrate your hard drive and also spread to USB keys and external hard drives.