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The comedian is showing signs of infection after Thames charity swim
Walliams, who ingested the river water during his 140 miles Thames crossing, is showing signs of of Weil's disease.
The disease is bred in the water and is potentially deadly.
The comedian is suffering from the symptoms, which include a rash, muscle pains, bad stomach and a fever.
Walliams has been already treated but the symptoms have come back, which has worried doctors as it's typical of Weil's disease to disappear and return days after.
"He is undergoing tests but at least he's in good hands" said a spokesperson.