Viruses can come from just about anywhere. I don't think there is an antivirus program that catches every one. Sometimes you need more than one program. More importantly, just because it's installed in a pc doesn't mean it's running when it needs to and updated as it should be. I know there are many people who think that just because it's installed they don't need to do anything. You need to be sure to stay on top of updates, keeping it set to automatic is best. Don't open suspicious emails - delete without opening. Even some emails that look legit aren't. Try a Malware program too. Take turns running them. I hear that Macs don't have nearly as many virus issues as PCs do.
Another thought, be cautious opening ads from some sites. We had issues at work from ads on CNN and the local newspaper's webpages.