But the fact that the Wikipedia project is funded exclusively by donors and the site never display ads also makes it a good litmus test for discovering whether one's machine is infected with certain types of malware.
"If you’re seeing advertisements for a for-profit industry or anything but our fundraiser, then your web browser has likely been infected with malware," Wikipedia's Director of Community Advocacy Philippe Beaudette pointed out in a recent blog post.
This usually happens when a specific browser extension has been inadvertently downloaded and installed by the user.
"Ads injected in this manner may be confined to some sites, even just to Wikipedia, or they may show up on all sites you visit," he says. "Browsing through a secure (HTTPS) connection may cause the ads to disappear, but will not fix the underlying problem."
One must disable the extension in question, but even after having done this, other malware could still be hiding on the computer, and then a malware scan is order.
Beaudette points out that ads seen on Wikipedia's site can have one final source - one that might not be malicious but is still annoying: the users' Internet provider, who injects them into web pages for profit.