Spokesperson Matteo Bruni previously said the results of tests conducted in recent days indicate the pontiff is suffering from a "polymicrobial respiratory tract infection" that needed a further change in his drug therapy.
The Vatican said that, despite his illness, Francis "read and worked" on Monday morning and said he was "touched by the numerous messages of affection and closeness that he continues to receive in these hours".
On Monday, the Vatican said his respiratory tract infection was presenting a "complex clinical picture" and he would need to remain in hospital.
The Vatican has cancelled Pope Francis's weekend engagements as the elderly pontiff battles a "complex" respiratory infection.
Francis, who has been pontiff since 2013, has had influenza and other health problems several times over the past two years.
