If left untreated, the bacterial blood condition can develop into septic shock.
Her publicist told the U.K's Daily Mail that Allen, who lost her baby just days before, is "responding well" and "continues to improve."
She lost the most recent baby, a boy with Sam Cooper due in January, last Monday. On Friday, she Tweeted, "Thank you for all your kind messages."
It's the 25-year-old singer's second such heartbreak – coming almost three years after she suffered a miscarriage during her relationship with musician Ed Simons.