The Japanese trojan virus installs itself on computers using a popular file-share service called Winni, used by up to 200m
It targets those downloading illegal copies of games in the Hentai genre, an explicit form of anime.
Website Yomiuri claims that 5500 people have so far admitted to being infected.
The virus, known as Kenzero, is being monitored by web security firm Trend Micro in Japan.
Masquerading as a game installation screen, it requests the PC owner's personal details.
It then takes screengrabs of the user's web history and publishes it online in their name, before sending an e-mail or pop-up screen demanding a
credit card payment of 1500 yen (£10) to "settle your violation of copyright law" and remove the webpage.