No other country in the world does not even come close in terms of the absolute number of infections.
According to Microsoft's latest Security Intelligence Report, which includes an analysis of vulnerabilities and exploits found on more than 600 million client machines, roughly 11 to 12 million PCs in the U.S. are infected with some type of malware - every quarter. This number is based on Windows systems as well as computers that run Microsoft's security software as well, but the distance to the second place is considerable: Brazil had just about 2.0 to 2.9 million infections per quarter in 2010, followed by China with 1.9 to 2.2 million.
The infection rate per PC shifts this story as Turkey showed especially infections per 1000 computers scanned. According to Microsoft 36.8 of 1000 PCs in Turkey were affected by malware in 2010. Other countries with high infection rates were Spain with 36.1. Korea followed with 34.8 and Taiwan with 29.7. Countries with low infection rates were Mongolia (1.3), Bangladesh (1.4), Belarus (1.6), Philippines (3.1), Austria (3.4), India (3.8) and Japan (4.4)