"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.
Immunologists argue over the significance of this, but a number of them believe that it will be difficult to come up with a vaccine that covers all these strains.
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.