People are being lured into opening worm-carrying emails by subject lines referring to alluring pictures.
Anti-virus company Trend Micro analysed infection rates, and found that the most common lure was sexually-explicit subjects.
The company found worms that spread in this way have included Sober, Netsky, MyDoom, Lovegate and Wurmark.
Adam Biviano, senior systems engineer at Trend Micro Australia, said that the virus writers needed to use some form of enticement to get the user to execute the file.
"[It's] trying to get a user to do something they wouldn't normally do -- therefore the virus comes through masquerading as some form of benign file," Biviano told iTnews.
He said people were continuing to get tricked into executing these files. "There's always a new angle the virus writers are trying that seems to appeal to human nature and gets executed."
According to Biviano, one of the other ways virus writers are trying to get people to open files, is by making it look as if it's from a system administrator, asking the user to take some form of action.
A statement from Trend Micro identified a range of subject lines that were used as part of this technique. These included fake notifications that an email account had expired, returned mail notifications, account suspensions and password update notification.
"These emails often include warnings such as: *IMPORTANT*, Notice: **Last warning**, etc., for added emphasis," according to the company. "Malwares that typically use this type of email subject line include: MYTOB, Sober and Bagle."