CSharpbucket.com

Placing an ad on your site can generate some income and add content to your site, but it can expose your visitors to unrelated content or malicious ads.

An article recently describes how some big companies, such as The New York Times, the BBC, and MSN, were targeted by malicious ads that encrypted visitor's computers until they bought a key to unlock it (ransomware).

There are plugins, such as AdBlocker, that will block a majority of ads and help protect you from unwanted content. It is one that I use and it's very effective at removing ads.