No OS is truly secure. You level of vulnerability is a combination of many factors, including the security model used by the OS, how securely critical OS functions are coded, how well users and system administrators adhere to best security practices, and how much malware is out "in the wild" that is trying to target the OS.
Furthermore, all desktop OSes compromise security in the name of functionality and/or ease of use to some degree. Windows (up through Windows XP) probably more so than others.
As the article indicates, OS X's reputation for being more secure is based at least in part on the fact that it simply has a small market share, making it a less attractive target for malware writers.
The years just pass like trains. I wave, but they don't slow down.
-- Steven Wilson