The date should be circled in black on your computer's calendar: the scourge of internet spam is 30-years-old today.

On May 3 1978, Gary Thuerk, a marketing man for DEC, a now defunct American computer company, sent what is thought to be the world's first junk e-mail. The unsolicited message was delivered to a mere 393 users of Arpanet, the US government network that would become the internet, but paved the way for trillions of future missives hawking counterfeit Viagra and dubious get-rich-quick schemes.

Today an estimated nine out of ten e-mails are unsolicited junk – accounting for about 100 billion messages every day and spam has become hugely lucrative, both for the racketeers that send messages and the firms that defend against them.

Spammers extracted $240 million from hapless internet users in the US alone last year, according to the FBI. Eclipsing that figure, Ferris Research, a technology firm, reckons that the global cost of fighting potentially dangerous e-mails will hit $140 billion this year.

Despite its current epic proportions, spam was something of a slow burner. It was not until the 1990s, when the internet and e-mail really took off, that the term became well-known. The phrase comes from the Monty Python sketch in which a group of Vikings chant the word "spam" ad nauseum in a restaurant where every item on the menu includes the eponymous luncheon meat.

For many the joke is wearing thin: one recent study suggested that the average UK worker will spend 3.5 years of their life dealing with "irrelevant e-mail".

In response, lawmakers have launched several counter-spam offensives and in 2005 Jeremy Jaynes, an American, became the first spammer to be prosecuted when a jury in Virginia recommended that he serve nine years in prison.

For the most part, however, the core cadre of leading spammers (it is estimated that 200 individuals are responsible for 80 per cent of the world's spam) have stayed well ahead of the authorities while beating some of the world's most brilliant computing minds into the bargain. Bill Gates's promise in 2003 that the problem "will be solved by 2006" was, it is now evident, overly optimistic.

In the spam arms race that has evolved, the spammers have evolved one key weapon. In 1978, an e-mailer had to tap into his computer the address of each recipient individually. Today, criminal gangs infiltrate huge numbers of public, government and corporate computers to turn them, unbeknown to their owners, into "zombie machines". The compromised computers are linked together to form "botnets", vast networks that can be harnessed to churn out e-mails automatically.

The power of the botnets is staggering, with some now rivalling the capacity of the world's largest supercomputers, machines typically used to explore the origins of he universe and cures for cancer.

Last year, an internet worm (a malicious piece of software which can itself be distributed via spam) called Storm infected an estimated 50 million home computers. Storm enslaved the PCs to create a giant botnet, which was used to send out billions of junk e-mails.

MessageLabs, the security group claimed that the Storm botnet had more capacity than IBM's Blue Gene/L, the world's fastest conventional supercomputer, which guards the US nuclear arsenal.

"If you sat them down to play chess, the botnet would win," a MessageLabs spokesman said.

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