A computer virus is a program that runs on your computer without your knowledge or permission and copies itself, often with harmful affects.
All computer viruses are manmade and some viruses are much more malicious than others.
A simple virus, which is relatively easy to produce, can make a copy of itself over and over again, which will quickly use all available memory and bring the system to a halt. Some viruses have even been known to add profane text to documents. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems. Anti-virus software allow the user to check their computer system for the known types of viruses and remove them. Macintosh How to download and install Disinfectant.