Each signature is characteristic of a particular virus, and when the scanner finds data in a file, email, or elsewhere that matches the signature, it concludes that it's found a virus. However, a virus scanner can only scan for the viruses it knows about. It's vital that you keep your virus scanner's signature file up to date, as new viruses are created every day. The problem actually goes a bit deeper than this, though. Look for Proactive Virus protection.
All human interaction involves exchanging data of some kind.
If someone weaves enough of that data together, they can identify you. Think about all the information that a person can glean in just a short conversation with you. In one glance, they can gauge your height, weight, and approximate age. Your accent will probably tell them what country you're from, and may even tell them what region of the country. If you talk about anything other than the weather, you'll probably tell them something about your family, your interests, where you live, and what you do for a living.