For a minimal investment of about $160, a single porn site operator can infect more than 20,000 computers with malware for use in cybercrime. The research team presented a detailed look at the online adult industry and mapped a lot of malicious Web activity to porn site operators. Using an automated tool to crawl a total of 269,566 URLs belonging to 35,083 porn sites, the group found the so-called free sites to be the most dangerous. The study was presented at the Workshop on the Economics of Information Security (WEIS 2010)
Source: https://threatpost.com/understanding-porn-malware-connections-060810/74076/