Mac system 1/5 malware is misleading

Information security company Sophos recently released a report saying that 1/5 of Mac OS X systems have malware. However, industry insiders said that this statement is misleading.
In most cases, malware for Windows systems will appear on Mac OS X. However, these malware were not activated and therefore did not exhibit malicious behavior, such as stealing information and modifying the system.
Most of these malware in Mac OS X come from spam, or malicious browser cache and cookie information, and exist in the user's email inbox. Since it is only for Windows systems, these malware cannot run on Mac OS X. However, in most cases, anti-virus software is not sure which malware really poses a threat to the system, but simply counts the amount of all malware.
In Sophos' report, about 1/36 of Mac OS X systems are really infected with malware. Most of these systems are infected with recent Flashback, while others have malware such as FakeAV and RSPlug.