Web users could be putting themselves at risk by surfing to some sites which contain over 18,000 different security threats each, says Symantec.

The security vendor has compiled a list of this year's 100 'Dirtiest Websites'. Just by visiting these sites, web users could see their PCs infected with viruses or even have their senitive personal data stolen by cyber criminals, even if they don't download or click on anything in particular.

Symantec said the average number of threats per malicious site is 23. However, all of the websites that made the top 100 list have on average 18,000 per site, with 40 per cent having over 20,000 security threats. The company also said that three quarters of the sites have been distributing malware for over six months.

As expected just under half of the websites offered adult content, but what is more surprising is that other 52 websites covered a random selection of topics such as deer hunting, catering, figure skating, legal services and electronics.

"There has been exponential growth in the number of online threats that are constantly evolving as cybercriminals look for new ways to target your money, identity or assets. In 2008, most new infections occurred while people were surfing the web," said Marian Merritt, Symantec's internet safety advocate.