Oh how many times I have heard this cry from mac users. Of course, the defenders will try to say how MacKeeper isn't technically a virus or malware, but the same problem that plagues the majority of PCs is the same problem for Macs. You can't create a system that prevents a stupid user from installing a program they shouldn't. The biggest security hole in any computer network is its users. Their lousy passwords and "clicky" habits will punch holes in a computer network every time.
MacKeeper is malware that played on the fears of Windows users who had switched the Macs and were used to running anti-Virus/anti-Malware and third-party utilities to keep their Windows machines clean and in tip-top running condition. So they were prime candidates for a heavily advertised Faux application that promised to do all of that. . . even though ALL of that is totally unnecessary on an OS X Mac. . . because they did not believe it wasn't necessary. Macs ARE superior in that way. They do not need any of those apps. They do not NEED anti-virus apps. Macs don't NEED anti-Malware apps. Mac certainly don't NEED third-party utilities to keep them clean and running in tip-top shape because the OS automatically does all that. In fact, MacKeeper turns OFF most of the built-in protections and utilities that do those those routine house keep tasks, including some that MacKeeper doesn't do itself!
These new Windows ex-patriot Mac users bought MacKeeper in droves, opening their machines to the Malware that is and was MacKeeper.
You will NOT hear me, a Mac defender, saying that MacKeeper isn't Malware. . . because I have been warning people against MacKeeper AS malware for years!
I will not defend stupid users either, but OS X Mac will generally keep even them from doing stupid things. It even WARNS them about MacKeeper.