Unix and Unix variants such as Mac OS and Linux have a reputation for being "virus free." However, though not as common as Windows viruses, they do exist. One reason for the rarity of Unix viruses is that the architecture of Unix systems was developed with the idea of user privileges, meaning that not all users can access all parts of the system -- this is something earlier versions of Windows did not employ. This simple design facet means that viruses cannot easily access administrative functions within the system to cause troubles.