In a challenge to the prevailing belief that Apple computers are immune to the sort of cyberattacks that plague WIndows-based machines, research firm Sophos has released a study claiming that one in five Macs have malware.

The report, released on Tuesday, is based on a "100,000-strong snapshot" of the millions of Macs that downloaded Sophos's free Mac antivirus software. The study found that 20% of Macs were carrying one or more instances of Windows malware.

Such malware doesn't cause symptoms unless the Mac owners run Windows on their machines, but it can be spread to others.

However, this doesn't appear to be solely a Windows-based problem. The report also found that 2.7% of Macs were infected with Mac OS malware. The majority of such Mac OS malware is composed of fake antivirus attacks, like the recent Flashback botnet. Mac owners can contract such malware by downloading email attachments, visiting rogue websites and unknowingly installing it via their USB drive. The chart below provides a breakdown of the types of Mac OS malware:

To avoid downloading such malware, Sophos recommends running an antivirus program and keeping it up to date, exercising caution about which links you click on, keep software patches current and keeping an eye out for email-based scams.

Thumbnail image courtesy of Flickr, Adam Fagen.