If you head to Wikipedia and spot an advertisement, be warned: It probably means your computer’s been infected with malware. The non-profit site is ad-free and funded by donors, so no ad you may spot there is legitimate.
The ad-injecting malware can manifest as a browser extension called “I want this,” which installs itself into the Google Chrome browser, according to a Wikipedia blog post; but there could also be other forms of the malware cropping up on additional browsers. As well, some Internet service providers (like those in Internet cafes or free Wi-Fi networks) may inject ads.
To prevent unwanted installation of browser add-ons, it recommends using the Web with a secure (HTTPS) connection, which can be automated using the HTTPS everywhere extension offered by the Electronic Frontier Foundation. See the Wikipedia blog post for more tips on preventing and removing the malware. And for advice and recommendations on security software, see our new buying guide at ConsumerReports.org.