There are several reasons to why mac and Linux are not affected by virus as much as Windows systems do. A couple of such reasons include -
1. Whenever you install any software on Linux, you do so by downloading it from distributor repositories and package managers. These repositories contain trusted software. Linux has no users who download any arbitrary software.
2. Linux does make use of many security features than rise on Windows.
3. Unlike Windows which has a wide range of users who are not necessarily experts in. On puts or software related matters. Whereas most of the Linux users are software geeks who know much about computers and related details. Also low market share in comparison to Windows makes it less lucrative for the hackers to develop virus.
But, that doesn't mean you can ignore the possibility of a threat. Using anti virus would be advisable despite them being less prone to virus attacks. Also please note that though there are no Mac virus, there is a possibility of malware infection.
Live....and Let Live!