John Carter knows what he's talking about. He writes, "In my experience, there are NO Mac viruses that Apple hasn’t taken care of, and that the only threats found anywhere are in some email message or in an application that is not blessed by Apple; in the latter case, it won’t be found in the App Store.
"If you have downloaded and installed an app from the Internet, such as Firefox or Chrome, that app may contain something that Sophos doesn’t like the looks of and declare it as suspicious or a threat. However, if you get the app from the source (e.g., Mozilla or Google), then you should not have any problems with that app. However, avoid links that claim to be the source or that offers extra features."
And John goes into detail in a report to be posted at the Prescott Mac User Group's website, www.pmug.us so you'll learn more about Sophos (www.sophos.com) and how to use it.