In its annual Computer Virus Prevalence Survey, the ICSA pointed to macro viruses, such as Melissa and the Class virus, as the main threat to corporations. Still, despite increases in the chance of infection, the cost of infections has been low. The results of the study are based on the replies received from some 300 corporate and government agencies. The ongoing survey has tracked the incidence of computer viruses since the beginning of 1997 and ended at the beginning of April 1999. The authors of the report were not available for comment at press time. The survey shows that computer virus infections have doubled each year for the past two years, reaching a total of 88 incidences per month for every 1,000 computers in February 1999, compared to 21 per month for the same period in 1997 and 32 per month in 1998. Almost half of the 300 companies polled had suffered a virus disaster, with 25 or more computers infected by a single virus. That's bad, but it could be worse. By far the majority (91 percent) of incidences resulted in less than an hour of down time for their servers, said respondents. Furthermore, cleaning up damage caused by viruses took less than 50 hours. Three major virus incidents this year have underscored the increase in the number of PCs catching the binary flu. In late March, a Word macro virus named Melissa spread using e-mail. A month later, a strain of CIH -- named for its purported author, Chen Ing-Hau -- deleted formatting information on hard drives in the U.S. and overseas. And, in early June, ExploreZip combined Melissa's e-mail attack with CIH's malicious way of deleting data. However, since the survey ended in early April, only the Melissa incident was included in the collected survey data. Even that small slice of data showed Melissa to have been an enormous problem for large organisations. Virus disasters that occurred after Melissa struck on March 26 were 38 times more likely to have been caused by Melissa than by other viruses. Furthermore, Melissa brought down three times more servers than other viruses infecting companies around the same time. Still, for the most part, the damage was contained. The survey found that Melissa did not cause more downtime than other viruses, and the cost of cleaning up Melissa was no different (statistically) than cleaning up previous infections. Yet, Melissa made evident another trend. For the first year, e-mail has replaced the floppy disk as the mode of infection for computer viruses. More than half of all viruses infecting computers in 1999 came through e-mail, up from 32 percent in 1998. Floppy disk infections, accounting for two-thirds of all incidences in 1998, fell to 39 percent in 1999. The study was sponsored by anti-virus and computer security firms Computer Associates International, Network Associates Inc., Panda Software and Symantec Corp.