Why Linux and Mac are almost virus free OS?

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.

No OS in software industry is truly virus free. Every thing has vulnerability.
Linux has a separate mode for handling kernal events. Even the admin can't access certain files which means no viruses can access the files stored in a normal way. Also in Linux system, the user privileges are very strict. So the virus run by the user can only affect user files and not the other files which means the effect of the virus in Linux is very less.

In windows OS, it is very easy to break the user privilege so the viruses are easy to design for windows interface.

Linux and Mac OS both are derived from the UNIX based architecture. And now they have their own kernel and the security architecture. So they are different in terms of kernel and process management compared to the Windows.

Both of these platforms are used in network infrastructure and high performance computing. So the viruses made for these platforms are very specific to these platforms. And though the security issues appear less compared to the Windows as most of the viruses and security issues are patched immediately.

Windows is popular on consumer desktop compared to Mac and Linux. So the viruses being developed by antivirus companies to make people use their products are more in number.

Whereas the consumers using Linux and Mac are more tech savvy and less likely to make mistakes while using these platforms. So the viruses developed for those architecture are different in terms of security penetration. And for Windows the same viruses written can be simple in nature and exploits could be severe as well.

In short the viruses being written for the platforms depends on the consumer using the platform. It also includes financial interests of the antivirus companies and the viruses are distributed into the dark web respectively.