How a virus works

The word virus is often being used as a common term for all malicious programs, but technically a virus is a program or code that attaches itself to a legitimate, executable piece of software, and then reproduces itself when that program is run. Viruses spread by reproducing and inserting themselves into programs, documents, or email attachments. They can be transmitted through emails or downloaded files and they can be present on CDs, DVDs, USB-drives and any other sort of digital media.

A virus normally requires action to successfully infect a victim. For instance - the malicious programs inside email attachments usually only strike if the recipient opens them. The effect of a virus can be anything from a simple prank that pops up messages to the complete destruction of programs and data.

In recent years viruses have been on the decrease. In January 2007, one in 119.9 e-mails, or 0.83 percent, were infected with viruses, while more than 20 percent of emails at times contained viruses five years earlier. The difference is partly due to virus attacks becoming more targeted and no longer occurring as one large outbreak. Also, there has been big increase in spam emails that contains links to download viruses.

The computer virus turned 25 in 2007. Long-suffering computer users would be forgiven for thinking that the first computer virus appeared in the mid-1980s, but the first virus actually predates the first IBM-compatible PC. Elk Cloner, which spread between Apple II computers via infected floppy disks, was released July 1982 and it was the first computer virus to spread in the wild.

Viruses had their heyday around the year 2000, with the Y2K scare. In 1999, the Melissa virus caught antivirus companies flat-footed and propagated rapidly. It was the first real outbreak of many of its kind that spread using Microsoft's Word and Outlook. A year later, the 'I Love You' virus caught the world by surprise. Lloyds of London estimated that the virus cost the global economy $10bn, making it the most expensive piece of malicious software to be unleashed to date. It was also the first time a computer virus became the day's top story for newspapers and television stations, marking a shift to mainstream awareness of computer viruses.

Nowadays, also mobile operators are starting to feel the pinch from viruses resulting from the increasing use of emails and Internet browsing on cellphones. Attacks on cellphones rose five times in 2006, with clients of 83 percent of mobile operators around the world having been hit, an industry study showed.

But mobile viruses are around 20 years behind those plaguing PCs, which translates into more than 300 virus variants targeting mobiles and smartphones, but around 400,000 such threats targeting PCs. In June 2004, a security company released details of a piece of mobile-phone malware that used Bluetooth to try to spread to other Symbian Series 60-based mobiles. That is believed to be the first case of a self-replicating mobile-phone virus and since then there has been a consistent increase in mobile viruses.