Microsoft Security Essentials tool counted 4 million detections on 535,752 distinct machines in first week alone. ID-theft related Trojans sit atop the detected category in the US. China has lots of potentially unwanted software threats, and worms (particularly Conficker) are very active in Brazil. Microsoft is sharing some very useful data on malware infections globally. In the first week, the tool counted four million detected malware on over 500,000 machines in the U.S. alone.
Source: https://threatpost.com/microsoft-malware-data-shows-trojans-rampant-usa-102109/72491/