Virtually all infections seen today are considered PUPS (Potentially Unwanted Programs). These are toolbars, adware, scam driver update programs, scam cleaner programs, etc. They do nothing good for your computer, and will cause a whole host of computer problems. Almost all computers I work on will have at least 1 PUP that was inadvertently installed.
PUPS get installed via legit free software. That is, the end user goes to install something and just keeps clicking next, next, next, without looking at the details of what is being installed. Even if the end user checks for what is being installed, the install options can be very misleading and tricky.
The "Unchecky" program aims to stop all that. I've tested it, and it works great.
https://unchecky.com/