Pernicious anemia is a disease characterised by the decrease in red blood cells (anemia) because of the lack of vitamin B12.
This lack is caused by the absence of specific cells in the intestine, gastric parietal cells, which are responsible for the uptake of vitamin B12 from the food. The loss of these cells can be causes by autoimmune diseases or gastritis. Vitamin B12 is needed for the synthesis of new cells, and a lack of it affects fast dividing cells, such as red blood cells, first.
Although the disease can be diagnosed and treated by life long injections of vitamin B12 replacements, no cure has been developed so far.
Sheldon mentioned in The Codpiece Topology that he had pernicious anemia in 1999 and played Super Mario. However, no further indications of any permanent treatment of such a conditions have been seen in the show so far. Also, Sheldon said "when I had pernicious anemia", while one would rather expect him to say "when I was diagnosed with pernicious anemia" because of the incurable nature of this disease.