One of Apple's marketing lines for Macs has been that they do not suffer from the same virus infections that PCs have. This clearly has nothing to do with Macs being more stable than PCs, but because fewer people own Macs than own PCs.
Cybercriminals are looking to infect as many machines as possible because this is how they create profit. It only makes sense that they would create viruses and other malware specifically designed for the Windows operating system. There are more Windows users, so there are more potential cyber victims and more profit to be made out of attacking Windows users. But that tide seems to be shifting.
Depending on what source you read, Apple's market share has begun to grow. At the beginning of the year it was reported that Apple's market share was growing while Window's was declining and that Apple then had 9.93% of the market. More recently Net Applications, a company that tracks operating system and market share by looking at results from search engines, reports that Apple's market share is now closer to 4.87%.
Either way, it is clear that Macs are gaining in popularity, and as their market share grows so will the number of attacks created for Apple's operating system. Already there are reports of hackers paying for hijacked Macs and we have to ask ourselves why.
Spammers (like all hackers) are playing a numbers game. Spammers know that only 1% of their messages will be opened and they will only be able to infect 1% of the people they send emails to. So to infect as many machines as possible they send out millions of messages knowing that if they send out a million messages with viruses they will infect approximately 10,000 systems. These systems could then be exploited to gain access to personal information (including financial information) and spread the infection maximizing the criminal's profit.
In the past it made sense to create viruses specifically designed to infect PCs because the majority of computers ran on Windows. Spammers and cybercriminals read the trends too. They know that Macs are becoming more popular. Not only that, I theorize that because Macs haven't suffered from the same onslaught of viruses and other malware that PCs have, Mac users aren't as vigilant as PC users and do not update their virus software as often.
Also, demographics come into play. Macs tend to attract older and younger computer users because they are seen as hip or easier to use. These same demographics are more likely to be careless about network and Internet security.
I believe cybercriminals are aware of this and are starting to create malicious programs targeted at Macs. If they are able to infect one Mac on a college campus (and the college doesn't have good network security technology in place that will quarantine an infection or block it all together), then there is a good chance the infection will spread. If Macs' popularity continues to rise we will see more and more viruses designed to infect Apple's operating system. Eventually, the lack of viruses will no longer be a differentiator for Apple.