How does a computer virus work? A virus is written by programmers to attach, overwrite, or otherwise replace another program on your computer in order to reproduce itself without your knowledge.
A virus can cause a number of problems on your infected device. It may quickly use all your computer's available memory — slowing down your system or bringing it to a halt. Viruses can damage data, destroy files, format hard drives, or make disks unreadable. A virus can enter your computer as an email attachment, in a downloaded file, or hidden on a zip drive or CD. It's usually not obvious that a virus is present on a website, in an email or elsewhere.