Dr. Jennifer Holter, who has been treating Billy at Oklahoma University Medical Center since the fall of 2008, says several immunologists have reviewed his case but the underlying reason for his immunodeficiency is still unclear.

CNN: iReport: 'Billy's Disease' baffles doctors

"I think when you have close to 25 immunologists look at you and they can't figure out what's causing the problem, you can call that pretty rare, " she says.

CNN: iReport: 'Billy's Disease' baffles doctors