Android is the most attacked mobile operating system according to the latest McAfee Threats Report.

The report which covers the Second Quarter of 2011 shows that the amount of malware targeted at Android devices jumped 76 percent since last quarter.  McAfee thinks that its comprehensive malware “zoo” collection will reach a record 75 million samples by the year’s end.

Vincent Weafer, senior vice president of McAfee Labs said that this year we’ve seen record breaking numbers of malware, especially on mobile devices. Overall attacks are becoming more stealth and more sophisticated, suggesting that we could see attacks that remain unnoticed for longer periods of time. High-profile hacktivist groups have also changed the landscape by drawing a line between attacks for personal gain and attacks meant to send a message, he said. The report cybercrime “pricebooks” that determine the going rate for large email address lists, and acts of hacktivism and cyberwar.

With an approximate 12 million unique samples for the first half of 2011, a 22 percent increase over 2010, this has been the busiest first half-year in malware history. This means that the grand total of total malware samples in McAfee’s database has reached approximately 65 million, and McAfee researchers estimate that this “Malware Zoo” will reach at least 75 million samples by the year’s end.

Android OS-based malware surpassed Symbian OS for the most popular target for mobile malware developers. While Symbian OS and Java ME remain the most targeted to date, the rapid rise in Android malware indicates that the platform could become an increasing target for cybercriminals – affecting everything from calendar apps, to comedy apps to SMS messages to a fake Angry Birds updates.

Apple now has become more a target for malware authors. Though historically the Apple platform has been unaffected by fake anti-virus (fake AV) software, it is now being affected.

Another malware category that is demonstrating recent steady growth is stealth malware. The tactic of hiding malware in a rootkit is used by cybercrminals to make malware stealthier and more persistent, and has seen this type of attack gain in prominence over the past year, with high-profile attacks such as Stuxnet. Stealth malware has increased more rapidly in the last six months than in any previous period, up almost 38 percent over 2010.