The Netsky virus still accounted for most email-borne infection in August despite the May arrest of its suspected author, security software firm Clearswift has said.
German teenager Sven Jaschan faces up to five years jail for computer sabotage, data manipulation and disruption of public systems, with a trial date yet to be announced.
Netsky was designed to disable the Bagle/MyDoom viruses, which hijack PCs for denial of service (DDoS) attacks.
Meanwhile, with an estimated 50 percent of global emails containing spam, Clearswift said spammers are becoming much more sophisticated in overcoming spam filters.
Five percent of all spam in August used Asian double byte characters (written in Chinese, Japanese and Korean), that spam filters based on single byte characters are unable to read, according to a statement from Clearswift Asia Pacific.
"The recent rise in spam based on double byte characters suggests spammers are seeing a high response rate in Asian speaking countries," said Peter Croft, managing director of Clearswift.
"It also indicates that more unsolicited mail is emanating from Asian countries, with spammers targeting Asia Pacific countries with localised offers."