<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by X-linked severe combined immunodeficiency ? [/INST] X-linked SCID is the most common form of severe combined immunodeficiency. Its exact incidence is unknown, but the condition probably affects at least 1 in 50,000 to 100,000 newborns. </s>