The 77-year-old, the only original cast member to reprise her role in the BBC's remake, said that she was "determined" to rejoin the period drama for its second run.
The Rose Buck star said: "I was absolutely determined [to get back]. Three weeks after the first episode I had a stroke and a heart attack and in three weeks I'd thrown myself out of the hospital.
"I said I will be alright and the main doctor said, 'All right, you can work again. But you can only work four hours a day'. And I said, 'Terrific!'"
Dame Eileen Atkins will be the only absentee in the new series, after the actress announced her departure in August.
Writer Heidi Thomas said Marsh's illness was an "emotional experience for all of [them], because [they] just adore Jean", adding: "I remember saying to Anne Reid (Mrs Thackeray), 'Rose cannot leave because it would be like the ravens leaving the tower, she's so integral to 165'."
Dismissing rumours that Atkins was unhappy with her scripts, Thomas insisted: "With Eileen it was all very mutual and not acrimonious in any way."
Meanwhile, shrugging off comparisons with ITV1's Emmy-winning Downton Abbey, she added: "It hasn't been part of our thinking while we make the show at all. We're so centred on what we do and I think we're all so passionate about these two families at this time in history, that's our story."
Upstairs Downstairs will return to BBC One in the spring.
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