Just what exactly is a virus? What kind is in your computer? How did it get there? How is it spreading and wreaking such havoc? And why is it bothering with your computer anyway?

Viruses are pieces of programming code that make copies of themselves, or replicate, inside your computer without asking your explicit written permission to do so. Forget getting your permission down on paper. Viruses don’t bother to seek your permission at all! Very invasive.

In comparison, there are pieces of code that might replicate inside your computer, say something your IT guy thinks you need. But the code spreads, perhaps throughout your office network, with your consent (or at least your IT guy’s consent). These types of replicating code are called agents, said Jimmy Kuo, a research fellow with McAfee AVERT, a research arm of anti-virus software-maker McAfee Inc.

In this article, though, we’re not talking about the good guys, or the agents. We’ll be talking about the bad guys, the viruses.

A long, long time ago in computer years, like five, most viruses were comprised of a similar breed. They entered your computer perhaps through an email attachment or a floppy disk (remember those?). Then they attached themselves to one of your files, say your Microsoft Word program.

When you opened your Microsoft Word program, the virus replicated and attached itself to other files. These could be other random files on your hard drive, the files furthest away from your Microsoft Word program, or other files, depending on how the virus writer wanted the virus to behave.

This virus code could contain hundreds or thousands of instructions. When it replicates it inserts those instructions, into the files it infects, said Carey Nachenberg, Chief Architect at Symantec Research Labs, an arm of anti-virus software-maker Symantec. Corp.