Yes, I've seen many differing causes of death for Mary. But wouldn't syphilis have affected Elizabeth too ( and she lived a long time), if it was the case for Edward and Mary( who died when young, the theory being it caused or contributed to their deaths)? Of course, this is also said to have been the case for James II and his kids (their lack of desdendents being proof, too, say some), and his daughters didn't live very long lives, but his son did. Henry VIII'S kids never had descendents, so there is no evidence with their own children ( or lack thereof, since only Mary tried to have children), there weren't any. Mary's failure to have children can be sufficiently explained by her age and health.