The biological mother discovers that her son has some sort of disease that is carried by women but only affects men. The biological mother then seeks out seeks out my character. Bio mom didn't know if the adopted child was a boy or a girl and wanted to let the child know that either she could be a carrier or he could be affected, depending on gender, of course.
Does that make sense? Is there anything that fits the bill?
XMEN Disease might take a while to diagnose and put you closer into the time frame of your question. It might be discovered after the child presents with frequent infections.