Virtually all of the "bad" stuff that gets into your device is stuff you let in yourself - usually through an email or email attachment - or in your haste to download a legitimate program you accidently give it permission to load something else that causes you a problem. You have to read EVERYTHING carefully before installing anything. Viruses don't appear all by themselves. But as consumers, we have been programmed to believe that installing anti-virus software on your device will protect you.
No so. After having one of our devices infected twice under the watchful eye of expensive Norton anti-virus protection, I canceled the subscription. Since then I have not used anti-virus software on any of my family's Windows computers for years, and we have not had a single infection. I don't open any email from an unrecognized sender. I regularly run a clean-up program and a malware program (both of them free) as well as making sure Windows Defender (Windows built-in security program) is running in the background at all times. I didn't make this decision lightly. I did a lot of research on the subject and asked a lot of questions of people who know a lot more about this than I do. I chose free malware and cleaning programs that came highly recommended by folks who use them regularly.
CBC News-Technology and Science recently posted an article on their blog entitled, Antivirus software is 'increasingly useless' and may make your computer less safe. It pretty much debunks the myth of anti-virus programs.
How you choose to protect your device is a personal decision - and I highly recommend you employ some system for keeping your computer(s) safe and running smoothly - but if you are spending up to $100 a year for an anti-virus program, you should read this article.