A virus could cost these companies thousands to fix lost data.
With this in mind the first anti-virus software was written to prevent these attacks.
Since then virus writers have found ways round anti-virus software.
One most common way is the 'trojan horse'. A trojan is a virus in disguise. It may appear to be another program such as a media player or a text editor but once it is executed it spreads a virus.
The first trojan was created in 1986. It was released as a popular program called PC write. The trojan horse is still one of the most common types of viruses today.
The worm is another common form of virus. Although it is not technically a virus, as it does not require a host. It can replicate itself without requiring a program to initiallly execute it.
Worms and viruses have become such a large threat mainly because of the internet.
The first large scale problem with worms was in 1987 when the christmas worm infected the IBM mainframe computers. This worm had the ability to replicate itself at over 500,000 times per hour and caused havoc on IBMs systems.
The following year in 1988 Robert Morris created the Internet worm. This was the first known 'Intenet Crisis' and it affected thousands of systems.
By 1995 hundreds of viruses were in circulation spreading from computer to computer over the internet.
With a large network such as th Internet that can connect any computer around the world it is bound to be the greatest threat for viruses.
Most viruses are spread via e-mail. Much like the 'I love you' virus of 2000 no one suspected the message to contain a virus so when the attachment is opened the virus is spread. The virus had the ability to send itself to everyone in the infected users address book allowing it to spread round the whole world quickly.
More advanced techniques have been introduced still such as viruses allowing remote control of a system. This can be serious as someone could have access to confidential information etc.