What are computer viruses?
Computer viruses are small programs that function by stealth and seek to enter your computer. Some are relatively benign, others are ruthlessly destructive.
Viruses are covert code – they do not want to be found. They execute their functions secretly, often shielded by layers of encryption.
Viruses are designed to replicate themselves, moving from one computer to the next as email attachments, through network ports that have inadvertently been left open, or via other means.
There are different kinds of viruses, each designed to disrupt computers in different ways. Some viruses automatically take control of your address book and forward themselves to all of your email contacts. Others sit patiently in the background, slowly corrupting your data. Still others are attached to specific programs – like Microsoft Word or Excel – and are triggered when you open a program, with the result that files and folders are immediately overwritten or erased. Whatever their form, viruses almost always damage the host computers they infect, often with fatal results.
The word virus is frequently used as an umbrella term to refer to several other common types of malicious code:
Worms are like viruses, except that they are self-propagating. That is, they do not need to attach themselves to a program or data within your computer to replicate. Instead, they use your computer as a host from which to further extend their infectious reach. Typically, viruses damage individual computers whereas worms disrupt bandwidth and network traffic. However, worms can carry harmful payloads, which they can deposit in infected computers. These programs can open a backdoor in your computer that will permit an unauthorized person to send millions of unsolicited emails. This is known as a Denial-of-Service (DoS) attack and normally targets corporate or government web sites.
Trojan Horses or Trojans are computer programs that, as their name suggests, conceal other programs within themselves. The shell program may appear legitimate (it might be a plug-in, or a piece of software downloaded from a popular site) but, when downloaded, it will seek out your computer password or personal banking information and send it to the author of the program. Like a worm, a Trojan Horse can also open a backdoor in your PC with similar consequences.
Increasingly, malicious coders combine aspects of all of these virus forms to create 'blended threats'.