Currency

From Frikipedia

Thanks to those who don't wash their hands, Currency is the chief spreader of communicable diseases in the Northern Hemisphere. Whether it's a coin or a bill, you can bet that atleast 4 types of bacteria and 2 viruses inhabit it's surface.

I once had a bill that looked like it had both Bird Flu and HIV-B. Lemme tell ya, I was more than happy to tip the bartender with that one!

Since there is almost no currency in the Southern Hemisphere, communicable diseases are less prevalent there.