On Wednesday evening, he fell ill at his Santaquin, Utah, home, according to a hospital statement.
She said only that he fell ill shortly after they ate dinner together.
The singer had to shelve 14 UK dates on the Symphonica tour late last year after he fell ill with pneumonia.
BBC: George Michael to re-schedule UK tour following illness
The couple separated in the 1980s, but his estranged wife was seen with him in recent months after he fell ill.
My father has always told me to get somewhere in life, but when he fell ill there was nobody else pushing me.
Immediately afterward, he fell ill with a succession of infections, including scarlet fever and pneumonia, which kept him hospitalized for almost a year.
One key witness the British officers hope to question is Andrei Lugovoi, a former Russian intelligence officer, who met with Litvinenko at a London hotel on the day he fell ill in November.
He was there in Ramallah when Palestinian Leader Yasser Arafat fell ill and later died, and he was there in Jerusalem when Israeli Prime Minister Ariel Sharon was left incapacitated after suffering a stroke.
Colin Davis got his big break in 1959 when Otto Klemperer fell ill and he took his place at the Royal Festival Hall to conduct a concert performance of Mozart's Don Giovanni, with Elisabeth Schwarzkopf and Joan Sutherland.
The 53-year-old presenter, who fell ill in January, said he was doing "a lot of physio" to help with his walking.
He cited a case of two children from the same day care centre in Houston, US, who fell ill with the same infection.
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