Malware
A specific component of anti-virus and anti-malware software, commonly referred to as an on-access or real-time scanner, hooks deep into the operating system's core or kernel and functions in a manner similar to how certain malware itself would attempt to operate, though with the user's informed permission for protecting the system. Any time the operating system accesses a file, the on-access scanner checks if the file is a 'legitimate' file or not. If the file is identified as malware by the scanner, the access operation will be stopped, the file will be dealt with by the scanner in a pre-defined way (how the anti-virus program was configured during/post installation), and the user will be notified. This may have a considerable performance impact on the operating system, though the degree of impact is dependent on how well the scanner was programmed. The goal is to stop any operations the malware may attempt on the system before they occur, including activities which might exploit bugs or trigger unexpected operating system behavior.
MindGeek
It was reported in April 2013 that visitors to some of the world's most popular (by Alexa rank) pornography sites risked malware being installed on their machines by malicious third party advertisements. The claims were made by security researcher Conrad Longmore, who writes a blog on malware, spam and scams. Longmore's figures were collected using Google Safe Browsing Diagnostics, which looks for harmful content on websites, and the statistics for two sites; xHamster and Pornhub were published by BBC News. The BBC article stated that based on Longmore's research, 5% of pages on xHamster (which has never been owned by Mindgeek / Manwin), and 12.7% of pages on Pornhub incorporated malvertising. MindGeek (still known as Manwin at the time) said in a statement that their own figures showed that 0.003% of the advertisements they served over the period that Longmore's data was collected contained malware, and they said that the advertisements they serve are monitored for malware and malware is immediately removed.