If you share files with PC users you can easily spread viruses for windows. And as stated above no Mac is immune. People have created malicious software for Macs in the past, so they can in the future.
Before we get too far ahead of ourselves here, we need to get back to reality. In the entire history of the Mac, there have been something like 26 Mac-only viruses. Most of those were developed during the reign of System 6 and the early reign of System 7. By the time MacOS 9 came onto the scene, there were fewer than one new Mac virus per year. With
Word 6, Microsoft enabled the first cross-platform virus. With
Office 98, Microsoft enabled the plethora of
VBA macro-viruses on the Mac. Because
VBA and Microsoft's older scripting language were sandboxed to the
Office environment,
Office macroviruses could do damage only within the
Office environment. By contrast,
VBA was the run of the system on Windows. Therefore,
Office macroviruses can damage anything on a Windows system.
A Mac may act as a relay and repository of Windows viruses. That said, your
Boot Camp installation of Windows and your installation of Windows within
Parallels or any other virtual environment are just as susceptible to Windows viruses as an Dell laptop.
Let's get back to the present. MacOS X has now been around for a decade. In all this time, there has not been one single MacOS X virus in the wild. There are zero reports of uncooperative MacOS X targets having been breached. Is MacOS X invulnerable to viruses? That I don't know. What I know is that no one has proven that it
is vulnerable.