Viruses, worms and trojans – they all do nasty things to your computer, but what exactly are they and how do they work? Paul Lomax explains.
Viruses
A virus is a software program that infects other programs and files on your computer, very much like their namesakes do with our bodies. They can often do serious damage, such as erasing your hard disk; or they may just be annoying and pop-up irritating messages telling you how clever their creator was. However, computer viruses rely on humans to spread them – you have to forward an infected file as an attachment to a friend for example. This is why you should always scan all files you have been sent and be very careful about opening attachments, even if you know the sender. Anti-virus software usually scans email attachments automatically, but you may be sent files by other means, such as instant messenger or IRC ('Internet Relay Chat'). Sometimes you don't even have to open the attachments in emails for them to infect your computer, which is where your anti-virus software should kick in, if kept up-to-date.
Worms
These are a bit smarter than a standard virus, the key difference being that they can spread by themselves, often by taking advantage of bugs in software. Most security patches from Microsoft are to stop worms from spreading, and it is vitally important you keep your operating system and software up to date (http://windowsupdate.microsoft.com). One recent worm, MS Blast, used vulnerabilities in Windows 2000 and XP and experts estimate caused millions of dollars worth of damage to both corporate and home users. Worms can also behave like viruses by infecting other files – the difference being the worm can send the files to all your friends and contacts via email or IRC without you knowing.
Trojans
Trojans, or Trojan Horses, are named after the legend of the siege of Troy (as seen in the recent Hollywood blockbuster) where the Greeks hide in a huge wooden horse in order to get into the city walls. Trojans do exactly the same thing as a virus or worm but use files with safe or tempting sounding names to coax you into opening them. Trying to trick you into opening something is known as 'social engineering'. Probably the most infamous is the 'Anna Kournikova' virus that spread by email and claimed the attachments were saucy pictures of the young tennis star. More recently the 'Sobig' virus sent fake emails that looked like they were from your ISP or IT department.
Getting smarter
The most effective and damaging computer infestations are now more commonly all three – they infect files like viruses, they spread themselves like worms, and they try and trick you into helping them spread, like trojans. Virus writers are trying all the tricks in the book at once to try and get past your defences, and it is a battle of good and evil between them and the anti-virus companies, to help keep you safe. But you can help, by updating your virus software regularly and ensuring you have all the software updates from Microsoft.
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Web User Essential Guide: Viruses
Viruses
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Claire Woffenden, Editor
