A virus is a computer program which can duplicate itself and 'infect' other computers automatically and can cause file deletion, system corruption and even cause computers to be formatted. With 10 to 15 new viruses discovered per day, the threat is growing exponentially with leading antivirus firm McAfee stating that more than 58,000 virus threats are currently in existence. Viruses can be spread via any transferable media whether in the form of floppy disks, CDs, email attachments or in material downloaded from the Web. The most common form of transfer is via email, in the form of an attachment which, when opened, activate a virus and infect a computer. Often, viruses are programmed to spread themselves by emailing a copy of themselves to all the other email addresses in an infected computer's address book.