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The actor and TV presenter, who usually hosts the BBC morning chat show with Gethin Jones, was absent this week after falling ill.
She was replaced by new Strictly: It Takes Two host and former Strictly Come Dancing pro Janette Manrara, as Jones explained to BBC Breakfast hosts Dan Walker and Louise Minchin while trailing the show.
He said: "Janette is in for Kym today. Kym is at home after testing positive for COVID so we will check in with her during the show."
Marsh, 45, had already been ruled out of Morning Live last week, as she was isolating after her son David and his girlfriend tested positive for COVID – and it seems she may have picked the virus up from them.
Calling into the show on Monday, she said: "The whole household went down like dominoes," adding that her 10-year-old daughter Polly had a high fever.
Marsh said they had been very careful after David became ill: "We took all the advice, he went and isolated in his room, we didn't have contact with him apart from obviously I was taking him food and drink, wearing masks, hand-washing, and we still all managed to get it, so it's amazing how transmissible it still is."
Asked how she was feeling, she said: "Not gonna lie, still rubbish, the cough more than anything is the thing... but I'm doing alright.
"I feel like we've been fairly lucky in terms of how sick we've been, so it could have been a lot worse, I'm sure."
The former Coronation Street star joined co-host Jones by video call last week to explain her family's health woes, admitting her son's girlfriend had been "really poorly".
She said at the time that she hadn't yet tested positive herself, explaining: "Yeah, so I wasn't well last week, I think you already mentioned that.
"I'd had a bad cold, and had two PCR tests both negative, but still was kind of coughing all over the place...
"I travelled home as always on the weekend, to be with my family, with my kids, and then over the weekend my son has tested positive for COVID. So not great."
She said of David: "He's okay. He's not doing too badly. So it's come from his girlfriend, his girlfriend is also self-isolating. She's been really, really poorly.
"David hasn't so far been as poorly as she has but today he said he's aching more today and he's coughing more as well.
"But up to now, he's doing okay, so fingers crossed he'll be alright."
It's been a tough few months for Marsh, who revealed in June that her beloved dad Dave had been diagnosed with incurable prostate cancer after putting off a check-up because he didn't want to put further pressure on the NHS during the pandemic.
Marsh and her dad made an emotional film for Morning Live about his diagnosis that left the presenter in tears live on air as she watched it back.
She said: "I'm sorry, it's so difficult for me to watch that. I'm proud of you, Dad."
Watch: Kym Marsh distraught over dad's incurable prostate cancer diagnosis