Viruses may also contain destructive code. These programs are generally designed to make changes to software that already exists on the system thus enabling them to replicate themselves, display messages, destroy data or software, or inhibit access by the user of the system. Computer viruses can be a serious threat to an organization since they usually result in loss of productivity. Access to personal computers is usually not restricted. This means that anyone could introduce a virus, intentionally or otherwise, to the environment.
Viruses can mutate as they propagate through an environment.
These types of viruses are very hard to detect and usually more harmful. Although techniques for detecting and removing known viruses have been developed and are relatively cheap to acquire, the problem will not disappear since not everyone protects and monitors their system or takes the steps necessary to prevent infection. Recognizing that a system is infected by a virus depends on the type of virus infecting your computer. Some viruses, such as the Friday the 13 th Virus, hide themselves on a system and then automatically activate themselves on a specific date. Others can prevent you from accessing files by altering directories and file tables.