To quote the University of Minnesota Extension brief on pigeons, "Pigeons may carry and spread diseases to which humans are susceptible. They are known to carry pigeon ornithosis, encephalitis, Newcastle disease, toxoplasmosis, salmonella, and several other less common diseases. Pigeons may also carry several different types of ectoparasites, such as mites, fleas and ticks."

In this publication from the U.S. Army Environmental Hygiene Agency (USAEHA) on bird and bat droppings, you can see that there are considerable risks involved in being around pigeon droppings. Pay particular attention to Chapter 3, "Representative cases of cryptococcosis and histoplasmosis":
http://chppm-www.apgea.army.mil/ento/tg142.htm

Obviously, pigeon droppings represent a health risk. If they are dry, it is especially important not to breathe the dust that will rise during removal. If the task seems daunting (and it may be, depending on the amount of droppings), we suggest you search the Yellow Pages for firms that specialize in cleaning buildings after fires, sewage spills, environmental damage, and the like, because they will have the specialized knowledge that will make this a safe activity.

Online Resources:
Bird Dropping Clean up Publications
How to Find a Qualified Professional Wildlife Control Operator