On January 2, computer users working on a software program in San Jose, California, found an unexpected message on their screens. It said:"Best wishes for a very happy New Year." Then it disappeared from the VDU.

Although that particular electronic note was harmless, it showed how vulnerable computer software can be to tampering or destruction by the so-called hackers. The hackers are computer addicts or vandals who manage to break into computer systems and networks.

Last month, the US defense department discovered that a foreign spy had infiltrated a network of computers used by the US Army. The hacker was looking for data on military equipment and plant, nuclear weapons and intelligent satellites. He probably did not gain access to any classified information but, if he had succeeded, he might have endangered the whole US defense system.

Many hackers try to access computers just for fun or for the intellectual challenge of it. But others try to steal private data or implant viruses malevolently.

A virus is a small software program that spreads through the memory and storage system of the computer, disrupting or destroying the data stored there. As its name suggests, a virus can move from one computer or disk to another, copy itself onto a computer's operating system, contaminate other software and cause damage beyond repair. This process goes on without the users knowing it. Some viruses can work very slowly and imperceptibly taking months to complete their work of destruction until it is too late to stop them and to repair the damage. Others can carry out their evil effects within a few minutes, for instance as soon as an operator switches on the computer and loads an infected program.

That is why such break-ins have increased fears about the security of the information held on computers from the small desk-top ones to the main frames and they may have incalculable consequences.