The problem with viruses that target Apple Macs is that very few Apple Mac users run any kind of anti-virus tools, unlike almost all Windows users. When a virus developer sends a virus after the Mac user community it often infects a huge percentage of Macs before it is noticed because there is no anti-virus scanner continually watching for viruses on about 95 percent of all Macs. They are basically wide open to virus infections.
In almost all cases, even when the virus becomes known to Apple, Mac users themselves are still blissfully unaware their computer is infected. The virus is happily doing its stuff, whether that be capturing username and password information for bank accounts and sending this information back off to its master, or just mischievous stuff like slowing down the screen video or causing the odd random system hang, whatever, until such time as Apple develop a bugfix for their operating system and then distribute it out. And this assumes that Apple ever find out about the virus.
Around 2000 the average number of viruses written to target Mac computers was about 50 per year. A paltry number compared to the estimated 2,500 written to target Windows computers in that year. Basically it was not really worth the time and effort for a highly skilled virus code writer to put the work into writing a virus that went after Macs. Of the estimated 200 million personal computers in the USA in the year 2000 only about 1.5 percent, or 3 million, were Apple Macs.
But as time moved on Macs have become a more interesting target for virus writers, both those doing it for ‘fun’ and those doing it to somehow make money from it (by taking some of your money).
According to Wikianswers there were one billion personal computers in the world at the end of 2010, but more interestingly the percentage of Macs had over double and was up to 3.3 percent—which means the number of Macs in the world was then up around 33 million. Over ten times as many as back at the start of 2000.
So now, with so many more Macs out there and people using them to do financial transactions on the Web they start become a much more interesting target for skilled virus code cutters; and especially those people that use viruses to somehow get some of your money without you noticing it (until it is too late). This is reflected in the huge increase of viruses per year that are now targeting Mac computers.
Even so, still very few Mac owners run any kind of anti-virus software. In a poll taken by GigaOM in 2010 (on this page here) almost 80 percent of Mac users said they did not run any anti-virus software.