There are other potential reasons, though, for a virus to cause your computer to be acting slow or in weird ways. And that leads us to a new segment – the reason virus writers would want to waste their time creating viruses in the first place.

The majority of viruses are still written by teenagers looking for some notoriety, Nachenberg said. But a growing segment of the virus-writing population has other intentions in mind.

For these other intentions, we first need to explain the "backdoor" concept.

The sole purpose of some viruses is to create vulnerability in your computer. Once it creates this hole of sorts, or backdoor, it signals home to mama or dada virus writer (kind of like in E.T.). Once the virus writer receives the signal, they can use and abuse your computer to their own likings.

Trojans are sometimes used to open backdoors. In fact that is usually their sole purpose, Kuo said.

Trojans are pieces of code you might download onto your computer, say, from a newsgroup. As in the Trojan War they are named after, they are usually disguised as innocuous pieces of code. But Trojans aren’t considered viruses because they don’t replicate.

Now back to the real viruses. Let’s say we have Joe Shoo virus writer. He sends out a virus that ends up infecting a thousand machines. But he doesn’t want the feds on his case. So he instructs the viruses on the various machines to send their signals, not of course to his computer, but to a place that can’t be traced. Hotmail email happens to be an example of one such place, Kuo said.

OK, so the virus writers now control these computers. What will they use them for? One use is to send spam. Once that backdoor is open, they bounce spam off of those computers and send it to other machines, Nachenberg said.

That’s right. Some spam you have in your email right now may have been originally sent to other innocent computers before it came to yours so that it could remain in disguise. If the authorities could track down the original senders of spam, they could crack down on spam itself. Spam senders don’t want that.

Ever heard of phishing emails? Those are the ones that purport to be from your internet service provider or bank. They typically request some information from you, like your credit card number. The problem is, they’re NOT from your internet service provider or your bank. They’re from evil people after your credit card number! Well, these emails are often sent the same way spam is sent, by sending them via innocent computers.

Of course makers of anti-virus software use a variety of methods to combat the onslaught of viruses. Norton, for instance, uses signature scanning, Nachenberg said.

Signature scanning is similar to the process of looking for DNA fingerprints, he said. Norton examines programming code to find what viruses are made of. It adds those bad instructions it finds to its large database of other bad code. Then it uses this vast database to seek out and match the code in it with similar code in your computer. When it finds such virus code, it lets you know!

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