Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by porphyria ?
    ### Response:
    The exact prevalence of porphyria is unknown, but it probably ranges from 1 in 500 to 1 in 50,000 people worldwide. Overall, porphyria cutanea tarda is the most common type of porphyria. For some forms of porphyria, the prevalence is unknown because many people with a genetic mutation associated with the disease never experience signs or symptoms.
    