<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by sialic acid storage disease ? [/INST] Sialic acid storage disease is a very rare disorder. ISSD has been identified in only a few dozen infants worldwide. Salla disease occurs mainly in Finland and Sweden and has been reported in approximately 150 people. A few individuals have been identified as having intermediate severe Salla disease. </s>