Spot on there IB and with your spelling,my wife has this condition and it's not nice to live with, it's an auto-immune disease, meaning the body attacks itself, in this case the stomach lining. Vit.B12 is essential for the formation of red blood cells so relies on stores of B12 in the liver,these run out after about 3yrs.Lack of B12 affects the blood's ability to carry oxygen throughout the body causing general weakness,Fairly rare condition so shouldn't happen too often in our pigeons