The Lenten season began for Christians this week, but one person was physically absent from religious observations: Pope Francis.

The head of the Catholic Church remained hospitalized on Ash Wednesday as he was treated for double pneumonia. The 88-year-old has been hospitalized with this infection for several weeks and has faced several setbacks in his recovery.

These types of infections are common this time of year, especially for elderly people, say Northeastern experts in gerontology, and recovery time is usually longer than it would be for a healthy person.

"There are many different types of respiratory infections at this time of year ... (when) people are inside," said Carla Bouwmeester, clinical professor of pharmacy and health systems science at Northeastern University. "They have a higher susceptibility, so we see pneumonia, which could be caused by multiple different factors."

Pneumonia involves fluid or debris in the lungs, said Stephen Wood, program director of the adult/gerontology acute care nurse practitioner program at Northeastern. Pneumonia can be contracted through bacteria or through other viruses like RSV and COVID-19. Pneumonia can also be caused by fungi or develop in health care settings where people are on a ventilator.

The fluid and debris in a person's lungs can affect that organ's ability to oxygenate and expel carbon dioxide, Wood added.

"So (your lungs) are less effective for those processes," Wood said. "Your oxygen levels go down and when your oxygen levels go down, you can see the other organs start to fail."

This can also lead to infection spreading throughout the body.

The Vatican said the pope has bilateral pneumonia, which means he is infected in both lungs. The cause is not clear. Francis does have chronic lung disease and had part of one lung removed when he was younger, a factor that Bouwmeester said may contribute to his slow recovery, which has been weeks.

"A very healthy 90-year-old may be able to recover from pneumonia relatively quickly, whereas conditions like COPD or asthma ... are going to make recovery longer," Bouwmeester said.