Over the past five years the number of virus infections reported by businesses has increased from 21 a month for every 1,000 PCs to 103 a month, research by ICSA Labs has revealed.
Virus disasters - defined as infections that hit at least 25 machines at once - cost companies an average of $69,000 (£48,000) last year, with some having to fork out up to $1m to repair the damage.
ICSA Labs' findings, which are based on telephone interviews with 300 US-based companies and government bodies, show that the vast majority of organisations regard the virus threat as worse or significantly worse than a year ago.
Their main concern is that virus attacks damage productivity, but the survey also discovered increasing concerns over the loss of data, file corruption and the difficulty of reading damaged files.
Most organisations reported minor but continuous problems, but a number of companies suffered major hits that resulted in significant downtime. The most pervasive virus in 2001 was Sircam, followed by Loveletter and Homepage.
New threats emerged in 2001 that are likely to set the scene for larger-scale virus attacks over the next two years. They include Internet worms, such as Code Red and Poizon-box, that have a rapid rate of dispersion, and worms such as Nimda, which spread through multiple channels to evade anti-virus protection.
Worms embedded in HTML code, which do not need to be hidden in e-mail attachments, are also a growing threat, ICSA said.