A computer virus is a piece of software designed to infect the target computer and usually involves a "payload" being dropped onto that computer. There are thousands of active viruses of varying types but there are also other infections too, such as spyware, malware and adware. These last three are not technically viruses but do similar harmful things to the computer.

Viruses have been around since the 1970s and in the first few decades were simply a form of graffiti and written to show off to friends. They often had very benign payloads that did very little harm. One of the best-known early viruses was called Cascade which, on a certain date, caused all the text characters on the screen to fall to the bottom of the screen:

More recently however, viruses are designed to make money and they accomplish this in two main ways. Firstly there are the viruses that resemble an anti-virus program and pretend that you have an infection. They then promise to fix all the issues if you pay them some money. The second method is to infect many hundreds or thousands of PCs (called "zombie PCs") and then blackmailing large organisations like banks to pay up or else face a concerted attack from these hijacked computers. Many computers attacking a bank's website can make the website unavailable to ordinary users for many hours or even days (this is called a "Denial of Service" or "DoS" attack).

It is therefore very important these days to have full protection, not just an anti-virus program running. A firewall and anti-spyware are also necessary and regularly scanning with these tools is a must too, so that any infection that may have bypassed the security can be caught before it spreads any further.

It is naturally a good policy to try to avoid viruses in the first place. The main things to avoid are opening unknown e-mail attachments and downloading unverified files (particularly "fun" things like videos, screensavers, so-called tools to fix or speed up your computer etc.). You should also make regular backups of your data in case a virus causes data loss (please see one of the other articles here about backing up your data).

The most important thing with an infection is to act fast. If you notice a sudden slowdown on your computer or popup windows or any other strange behaviour, your computer may well be infected. The quicker the infection is removed the better as if they are left on your computer, they can take many hours to remove. The worst infection I have ever dealt with took 6 (yup, six) hours to fully remove and this was simply because it was ignored for too long. Fortunately though, most infections are much simpler and quicker to clean!

Please get in touch if you need advice on adequate protection for your computer (security software is not all equal by any means) as prevention is infinitely better than cure.