It used to be that you would buy a Mac and avoid malware and viruses. That doesn't work as well in recent years as Macs have become a more attractive target. Now, a new trojan is spreading that specifically is aimed at Macs.
Under the cover of a browser plugin, media player, video-quality enhancement program, or download accelerator, the malware prompts you to install the trojan. Once you download the that app, you are asked to install “Free Twit Tube” or something similar. Don't do it.