Actually, some of the first and most destructive viruses were initially written for Mac, experts say -- back in the 1980s when Mac still had sizable market share. Viruses for Macs dropped significantly in the mid 90s, along with Mac's market share and credibility in the marketplace. But the viruses have since experienced a resurgence as Mac gained popularity after 2001 with its Tiger, Leopard and now Snow Leopard operating systems.