Increased use of stealth techniques such as rootkits is leading to fewer reports of new viruses, according to a study by net security outfit VeriSign iDefense.

Since 2003, VeriSign iDefense has been collecting and recording information on every uniquely identifiable malicious code, using public and private resources. Discovery of malicious codes was steadily growing between 2003 and 2005, but from January to June 2006 VeriSign iDefense noticed a significant downward trend each month.

Its conclusions are markedly different from those of anti-virus vendors, who report a decrease in mass-mailing worms but an increase in more targeted attacks.