What Is a Computer Virus?
A computer virus is a program – a piece of executable code – that has the unique ability to replicate. Like biological viruses, computer viruses can spread quickly and are often difficult to eradicate. They can attach themselves to just about any type of file and are spread as files that are copied and sent from individual to individual.

Besides replication, some computer viruses have something else in common: a damage routine that can deliver the virus payload. While payloads may only display messages or images, they can also destroy files, reformat
your hard drive, or cause other kinds of damage. If the virus doesn’t contain a damage routine, it can still cause trouble by taking up storage space and memory, and downgrading the overall performance of your computer.

Top Viruses of 2009 : Based on Ranking


Several years ago most viruses spread primarily via floppy disk, but the Internet has introduced new virus distribution mechanisms. With email now used as an important business communication tool, viruses are spreading faster than ever. Viruses attached to email messages can infect an entire enterprise in a matter of minutes, costing companies millions of dollars annually in productivity loss and clean-up expenses.
Viruses won’t go away any time soon. The number of computer viruses in circulation has reached one million for the first time, according to a report by a leading security firm. With numbers like those, it’s safe to say that most organizations will deal regularly with virus outbreaks. No one who uses computers is immune from viruses.

Virus Types: The majority of viruses fall into eight main classes:

Boot sector - Program viruses- Stealth viruses
File infector - Polymorphic viruses - Active X
Multi-partite - Macro viruses