Both Linux and MAC have such a small percentage of the total computer market that the Virus and Malware writers do not target them.
I don't think this entirely explains the massive number of viruses on Windows. Many of them wouldn't even work on Macs or Linux because they don't have the web browser, and thus ActiveX, so tightly integrated into the system. Earlier versions of Outlook and Outlook Express were almost as bad. How many email programs on Macs or Linux automatically ran script files attached to emails as soon as the email as displayed in the preview pane? Having services running by default that most users don't need (Messenger for example) is another security problem that doesn't happen anywhere near as often on other platforms.
Microsoft doesn't get much sympathy from me for all of their security problems. If they had put even half of the effort into security as they did into adding new "features", it wouldn't be such a mess. One MS VP admitted it wouldn't be until around 2011 that they had fully secured the system.
User education will help, but it is getting somewhat ridiculous that we have to ask people to install anti-virus, firewall, Spybot, Ad-Aware and other security related problems.
Linux and Mac are not immune, but being based on Unix designs which included security from the start, they are far less susceptible to virus attacks. This is part of the reason why I will push people to at least look Macs if they don't absolutely require some Windows only program.
goates