A virus will function as an element that will constitute damage to the hard disk contents or interfere with the normal operational executions of the computer system. This will cause the computer to be unable to function properly until the virus is eliminated and the overall system is overhauled and rebooted.
Let me explain... A virus program is usually able to replicate itself and this too is an added problem once the virus latches on to a system. Progressively getting out of control, the virus will attempt to cause as much damage as possible before it can be detected and eliminated. The replication is usually intentional and designed to act just like a Trojan, thus causing the unsuspecting user being caught off guard. If a file that contains a virus is opened, or copied onto another computer, then the other computer will also become infected and this process is repeated every time the file is opened and downloaded onto other systems.