Dr. Charles Gerba, a microbiologist from the University of Arizona
In terms of bacteria per square inch, they found that the phone receiver was the filthiest - 25,000 (probably because many people can share the same phone). This was followed by the desktop at 21,000, the computer keyboard at 3,300 and the computer mouse at 1,700. The least contaminated surface was the toilet seat with only 49 bacteria per square inch - making it about 50 times cleaner than the desktop. Gerba says that, for bacteria, the "desk is really the laptop of luxury. They can feast all day from breakfast to lunch and even dinner."
Full story here
http://www.abc.net.au/science/k2/moments/s1143577.htm