Why Do Doctors Wear White Coats?

http://blogs.wsj.com/health/2009/06/16/ama-the-white-coats-a...

Personally I don't get it. If white coats harbour diseases, doesn't it follow that whatever else they wear will have the same problem? Presumably one of the reasons coats were white were to show up dirt so that they would be washed more promptly. In what way is wearing a black t-shirt (or whatever) more hygienic?