From what I recall Mary writing in SUPREME FAITH, no one ever told her what Jean's illness was and in Mary's opinion, it seemed to occur when she was unhappy. So that's why she was suspicious.
In one of the Motown books, Cindy mentioned something about Jean having a headache in Vegas one night and it being almost impossible to get her to go on stage. She went on to say that she and Mary weren't used to that, even saying that Diana was the type who could have a major operation and still be on stage that night.
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