The damage caused by four significant new viruses in recent days has highlighted how poor patching and lax security are making life easy for virus writers.

The worldwide alert over the Blaster worm, and its subsequent infection rate, has flagged up the failure of many IT managers to ensure that systems are properly patched.

A new variant, Blaster D, is still taking advantage of unpatched systems.

Meanwhile the "vigilante" virus, known as Nachi, seems to have been created to do the IT manager's job by automatically patching vulnerable systems before deleting itself.

And the SoBig F worm, spammed to millions in recent days, is now the most common virus in the wild, according to MessageLabs.

"The writers of SoBig are being very stealthy. This is obviously an ongoing project, and we'll see more variants sooner rather than later," warned Pete Simpson, manager of Clearswift's ThreatLabs.

"Infected machines are being used to spam the virus around and you really need to take action now to block the spread further."

Graham Cluley, senior analyst at antivirus vendor Sophos, added: "What's concerning is that IT managers could protect themselves very simply.

"Blocking all .PIF files at the firewall would stop SoBig dead in its tracks. It's mystifying that more people aren't taking these precautions."