MacKeeper tells me I have a virus. Is this true?

Do not install MacKeeper (and how to uninstall it if you have):

https://discussions.apple.com/docs/DOC-6221

How to uninsrall it:

http://applehelpwriter.com/2011/09/21/how-to-uninstall-mackeeper-malware/

It contains a major security flaw, recently noted:

http://www.macnn.com/articles/15/05/08/contentious.utility.ignored.apple.guideli nes.created.zero.day.exploit.128538/

There is currently a class-action against MacKeeper:

http://www.macworld.com/article/2927032/ads-for-mackeeper-refunds-will-run-on-fa cebook.html#tk.nl_mwbest

(Please note that references to the original developers, Zeobit, also now refer to Kromtech Alliance Corp, who acquired MacKeeper and PCKeeper from ZeoBit LLC in early 2013.) And in that context the foregoing also applies to their latest offerings,Advnced MacKeeper, AdwareBuster and AdvancedMacCleaner ,which were quickly found to not identify much of the adware around.

As zfJames indicates, the malware you have is MacKeeper. Get rid of it.

See this article for additional information:

Effective defenses against malware and other threats

If you're concerned about it, you can download a safe program, like Malwarebytes (free at: Malwarebytes | Free Cyber Security & Anti-Malware Software), to check your system.

That sounds like a fake pop-up ad. Just ignore it.

Absolutely not true. NEVER download MacKeeper.