http://www.thebulletin.org/columns/laura-kahn/20080107.html
First paragraph only excerpt:
It's estimated that about 10 million people in the United States (3.6 percent of the population) are immunocompromised. But that's likely an underestimate because it only includes those with HIV/AIDS (diagnosed and undiagnosed), organ transplant recipients, and cancer patients; there's a sizable population that takes immunosuppressive drugs for other disorders such as rheumatoid arthritis and inflammatory bowel disease.