Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by sialic acid storage disease ?
    ### Response:
    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.
    