Do you know what an "ad injector" is? Don't worry if you've never heard of it, because it's a fairly new term. In fact, it's only really been around since the fiasco with Lenovo and Superfish earlier this year.
If you didn't hear about that, you can click here for the full details, but the summary is that Lenovo was pre-installing the Superfish program on some of its computers. The program watched your activity to serve you relevant ads while you were browsing. Unfortunately, it also opened the door for hackers to direct you to malicious sites without setting off any warnings.
Lenovo took care of the Superfish problem, but ad injectors are now on everyone's minds. These are often third-party programs that either put extra ads on pages while you're browsing, or launch popup ads while you're using your computer.
According to Google, 5.5% of its users (that means millions of people) have ad injectors on their computer. And nearly 30% of these injectors were malicious or serve malicious ads.