The number of unique malware samples broke the 75 million mark in 2011, McAfee notes in its fourth-quarter threats report for 2011, which also looks back at annual global trends in malicious code activity.Slideshow: The Future of MalwareAccording to the McAfee report, total mobile malware samples identified spiked upward to more than 400 in the fourth quarter of last year, a sharp rise from each of the year's three previous quarters that had seen only about 100 or fewer. "It's primarily Android," says McAfee researcher Adam Wosotowsky, adding that the samples are being discovered mostly outside the official Android store, not in it.MORE: 13 security myths"Much of the Android malware has been for-profit SMS-sending Trojans, which benefit cybercriminals by hijacking phones to send messages that cost their owners money," the McAfee Threats Report: Fourth Quarter 2011 report concludes.The reports says rooting Android devices "has become easier with the availability of apps that combine vulnerability exploits." Combining root exploits with malware is a technique "that has long been popular in the PC malware world and is a great example of transitioning what works on one platform to a new one."LOOK BACK: 15 worst Internet privacy scandals everThe McAfee threats report for Q4 2011 also says it appears that the amount of fake antivirus software (also referred to as fake alert or rogue security software), which is created to lure victims into buying phony A\/V or downloading Trojans, is on the decline.Apple Macintosh users have generally been far less targeted for malware than PC users over the years. Mac malware showed a huge spike upward the second quarter of last year in the number of unique Mac OS malware samples discovered, rising to almost 300 unique samples in June 2011, but then dropped off sharply, with very few discovered since, month over month. "In contrast to the second quarter spike, fake AV on the Mac is once again practically nonexistent," the McAfee reports notes.Ellen Messmer is senior editor at Network World, an IDG publication and website, where she covers news and technology trends related to information security.Read more about wide area network in Network World's Wide Area Network section.