According to Consumers Reports  If you head to wikipedia and you see an ad, beware you have more than likely been infected with malware. Wikipedia is a non-profit site with funding coming from donors, it isn't suppose to have ads. 

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. 

To Remove "I want this" on Google Chrome 

• Open the options menu via the “pipe-wrench” icon on the top right, and choose Settings.
• Open the Extensions panel and there is the list of extensions installed.
• Remove an Extension by clicking the Remove button next to an item.