Email security company Frontbridge Technologies said it blocked more than 40 million email-borne viruses in July, with Netsky.P representing one quarter of all virus and worm traffic.
MyDoom.0, which was discovered on July 26, spread very quickly, with FrontBridge blocking 200,000 copies in the first 24 hours.
The company warned that users are still being fooled into opening infected emails which use a variety of tricks, including the promise of provocative pictures or masquerading as patches or virus removal tools. MyDoom worms, for example, often arrive disguised as warnings from an ISP or system administrator or as bounced and rejected messages.
Postini, another email security company ranked Zafi in second place last month and put Mydoom.o in fifth place. Antivirus vendor Sophos ranked Zafi as the most prevalent threat in July.
Frontbridge warned that email worms, such as the Sobig family, can leave spam proxies on infected systems even after the worm itself is removed. This has helped boost the growing army of unwitting users whose machines, known as zombies, can be hijacked to launch massive denial-of-serice attacks.