The Pope has been diagnosed with bilateral pneumonia but is in a "good mood", the Vatican has said.
It comes after it cancelled Pope Francis's weekend engagements.
In a statement, the Vatican said: "Laboratory tests, chest X-rays and the clinical conditions of the Holy Father continue to present a complex picture.
"The polymicrobial infection, which arose in a context of bronchiectasis and asthmatic bronchitis, and which required the use of antibiotic cortisone therapy, makes the therapeutic treatment more complex.
"The chest CT scan that the Holy Father underwent this afternoon, prescribed by the Vatican health team and the medical team of the Fondazione Policlinico 'A. Gemelli', demonstrated the onset of bilateral pneumonia that required further pharmacological therapy.
"Pope Francis is in a good mood, however."
The statement went on to say he "alternated rest with prayer and reading texts" during Tuesday and that he "gives thanks for the closeness he feels at this moment and asks, with a grateful heart, that we continue to pray for him".
Bilateral pneumonia is inflammation of both lungs, usually caused by an infection, according to the NHS website.
The 88-year-old, who as a young adult developed a case of pleurisy and had part of one of his lungs removed, has been in Rome's Gemelli hospital since Friday for bronchitis treatment.
On Tuesday, the Vatican said: "The Pope had a quiet night. He rested. This morning he had breakfast and read some newspaper."
A further update is expected later today.
On Monday, the Vatican said his respiratory tract infection was presenting a "complex clinical picture" and he would need to remain in hospital.
That evening, the Vatican said the Pope was in a "stable" condition with no fever.
Spokesperson Matteo Bruni previously said the results of recent tests indicate the pontiff is suffering from a "polymicrobial respiratory tract infection" that needed a further change in his drug therapy.
Polymicrobial infections are caused by a mix of viruses, bacteria, fungi and parasites.
The Vatican said despite his illness, Francis "read and worked" on Monday morning, adding he was "touched by the numerous messages of affection and closeness that he continues to receive in these hours".
Concerns have been growing about the pontiff's increasingly frail health.
Francis, who has been pontiff since 2013, has had influenza and other health problems several times over the past two years.
Pilgrims arriving at the Vatican this week have offered their prayers that the pope would recover soon.
"We certainly wish for him to get better very quickly," said Reverend Tyler Carter, a Catholic priest from the US. "He is our father and our shepherd, and so we want his continued health and blessing."
(c) Sky News 2025: Pope Francis has 'bilateral pneumonia' but remains in 'good mood', Vatican says