Explaining the situation to stand-in host Gethin Jones, Alex, 43, said: "I am absolutely gutted that I am not there but what's happened is a friend of mine, somebody outside work who I saw a few days ago, has tested positive for coronavirus.
WATCH: Alex Jones pays emotional tribute to late colleague
"So we have done the right thing as a family and we are just isolating."
As a precaution, Alex revealed she took a test to see if she had contracted the disease, which thankfully came back negative.
Alex appeared via video call from her home on Friday
She continued: "Now I have taken a test and thankfully it is negative - by the way my friend is fine as well - but we are staying, we are following guidelines."
Revealing she will miss all of next week's show, Alex added: "I will probably be back at work the week after next."
Finding the silver lining though, Alex – who shares sons Teddy, three, and Kit, one, with husband Charlie Thomson – admitted there were perks to staying at home.
Alex will miss one week of The One Show
"There are definitely upsides to it," she said. "Charlie is back with the children, you can probably hear them just above me now, and I will go and read them some stories in a sec, which is lovely.
"The thing is, it is what it is. We are all in the same boat, so that's it really. But I miss you all. I miss the crew as well."
According to the NHS, people are advised to self-isolate if someone they live with, or who is in their support bubble, has symptoms or has tested positive for COVID-19, or if they have been told to self-isolate by NHS Test and Trace.
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