The names at the top of list are always changing as familiar ones fade and darkhorse newcomers take their place. In the Top 10 now are such notables as WM/Concept, WM/Wazzu, Stealth B and AntiCMOS.

They are computer viruses, ranked each year by frequency of occurrence by the International Computer Security Association. Watch for a change in next year's figures: WM/Concept, for instance, could get edged out. It accounted for about two-thirds of infections among the Top 10 viruses in early 1997; this year it had dropped to one-third.

You might think that viruses - snippets of programming code that spread surreptitiously from machine to machine - are being eradicated, what with all the anti-virus software one finds on a typical computer these days.

But paradoxically, the association's studies show that detected infection rates are continuing to rise, and the people who amuse themselves by creating new viruses still are very much at it.

A survey of 300 North American organisations earlier this year that operate more than half a million computers found that 31.4 virus encounters per 1,000 computers had occurred in the month before the survey. The corresponding figure for 1997 was 28; for 1996 it was 14.4.

Part of the rise may be due to better reporting, virus experts said. Despite their fearsome reputations - some can wipe out everything on a personal computer - many viruses do no real harm.

In the old days, before protection software was commonplace, a virus could pass undetected in and out of a computer. Now viruses can set the machine beeping when the virus scanner discovers them, and they officially enter the statistics.

As reported infections rise, the types of viruses are changing as well. We're seeing more "macroviruses" like WM/Concept, which take advantage of features in Microsoft Word and other applications that allow you to set up your own automated commands.

These generally are less potent than standard types of viruses, which are miniature programs, but they can still do some nasty things such as drop extraneous words into text or change a formula on a spreadsheet.

As email and Internet use expand, transmission methods are changing as well, the association's study showed. The typical method of exchange of infected floppy disks remains the most common, though viruses hiding in attachments in email are catching up.

Fast in pursuit of the new critters are virus researchers. Their goal is to find and catalogue previously unknown viruses that exist "in the wild", that is, in the world's supply of personal computers.

Most computer viruses exist only in laboratories. Of the nearly 20,000 that researchers have identified, only several hundred are actively moving from machine to machine. The rest have been eradicated from the wild, or were never there, having been sent directly to the labs by their creators.