She began by degrees to slacken in her constant coming to prayers and to sacrament, in which she had been before that regular, almost to superstition. She put that on her ill health: for she fell into an ill habit of body, which some imputed to the effect of some of the duke's distempers communicated to her. A story was set about, and generally believed, that the earl of Southesk, that had married a daughter of duke Hamilton's, suspecting some familiarities between the duke and his wife, had taken a sure method to procure a disease to himself, which he communicated to his wife, and was by that means set round till it came to the duchess, who was so tainted with it, that it was the occasion of the death of all her children, except the two daughters, our two queens; and was believed the cause of an illness under which she languished long, and died so corrupted, that in dressing her body after her death, one of her breasts burst, being a mass of corruption. Lord Southesk was for some years not ill pleased to have this believed. It looked like a peculiar strain of revenge, with which he seemed much delighted.--From History of his own Times--
I believe they are talking about syphilis.