While it is entirely possible for Apple computers to get viruses, ultimately the answer really isn't that simple. When compared to the number of viruses that users of Microsoft's Windows operating system can pick up, Apple computers get very few viruses. As any user of Windows can attest, it can almost be hard not to pick up a computer virus sometimes, especially when using the Internet. This is due to a variety of very specific reasons.
The main reason that Microsoft Windows users pick up a lot of viruses while Apple users pick up very little is that Windows has a vastly higher user base. This means that there are simply more Windows computers available than there are Apple computers. This makes Windows users an easier target due to their superior numbers. Also, a lot of virus makers are said to be more familiar with the Windows operating system and therefore only have the knowledge to create a virus to affect these machines.
Another reason why Apple computers pick up very few viruses is their stability. Mac OS X, which is the operating system that has run on most Apple computers for the last few years, was programmed using what is referred to as a "Unix Kernel." This means that Apple took the basics of an older operating system (Unix) and improved upon it to create Mac OS X. Unix, despite its age, is still one of the most secure operating systems ever created.
The final reason why Apple computers pick up very few viruses is simply because most of the tools someone would use to create a computer virus are designed to be used with the Microsoft Windows operating system. This doesn't stop at viruses--most of the tools used to create malware and spyware (which are other types of malicious software) are also intended to be used with Microsoft Windows.