This article claims that there exist more than 100 malware targeting the mac.
http://www.ft.com/cms/s/0/c79b814e-a364-11dc-b229-0000779fd2ac.html?nclick_check=1
http://www.ft.com/cms/s/0/c79b814e-a364-11dc-b229-0000779fd2ac.html?nclick_check=1