This as to do with D'Adamo's concept of blood type relation with diet and general well being - more specifically with secretion of antibodies. Below is an excerpt.

It is apparent that Non-secretors are in disadvantage healthwise as they are more prone to infections and allergies.
"In a simplified sense, a Secretor is defined as a person who secretes their blood type antigens into body fluids and secretions like the saliva in your mouth, the mucus in your digestive tract and respiratory cavities, etc. Basically what this means is that a secretor puts their blood type into these body fluids. A Non-secretor on the other hand puts little to none of their blood type into these same fluids. As a general rule, in the U.S. about 20% of the population are Non-secretors (with the remaining 80% being Secretors).


Candida infection

Based upon this tendency of Non-secretor saliva to not only fail to prevent attachment of Candida., but maybe actually promote the binding of Candida to your tissue, we would expect that research would show higher tendency to Candida problems among Non-secretors. This is what we find to be true. Non-secretors are much more likely to be carriers of Candida and to have problems with persistent infections. Blood type O Non-secretors might be the most affected of the Non-secretor blood types, since Candida also appears to have an easier time colonizing (attaching to) the blood type O antigen."

http://www.dadamo.com/napharm/store3/template2/secretor.htm