After the first jam-packed day of his apostolic journey to several Latin American countries, Pope Francis seemed little affected by the long flight or full timetable that kept him working throughout the day, with an open-air Mass with as many as a million in attendance, meetings with bishops, indigenous groups, and students, as well as a visit to a historic Jesuit church.

In a press briefing, papal spokesman Father Federico Lombardi was asked about Francis’s health. He answered that “the Pope himself is impressed by the energy he has.”

“The Pope’s health isn’t normal,” Lombardi said. “He has an energy that isn’t natural. He himself says it comes from God.”

“It’s always surprising what the Pope can do at his age,” Lombardi added, noting that several people in the papal entourage woke with headaches on Monday due to altitude sickness, but the Pope was fine.

“He has said it’s God’s way of helping him do his ministry, his service,” Lombardi said.

The 78-year-old Pope only has one full lung and has commented that he believes his pontificate will be a short one. Last March, Francis said he might have two or three years left.

Still, by all accounts, the Pope is going strong, something that was observed during his January trips to Sri Lanka and the Philippines.

Remarking then on the Pope’s endurance, Father Lombardi said that “we are always very surprised that a man of his age can do what he is doing.”

The Pope himself said he was surprised that he had the strength to do “so much.”

According to Lombardi, Francis believes that it is the grace of the office God has given him.

He said that the public becomes energized when they see the Pope, but that the relationship is reciprocal: the Pope also draws energy from the people who come to see him.

Lombardi said that he once asked the Pope about his vitality during their visit to South Korea: “This mission to service and pastor gives me strength that I never thought I would have,” Lombardi recalled the Pope telling him.

Manila Archbishop Luis Antonio Cardinal Tagle also remarked that though Pope Francis gets tired, he is easily reinvigorated whenever he sees the crowds who have come out to see him.

“At the end of every event I would ask him, ‘How are you? You feel tired?’ And he is very frank and would say, ‘Yes, a little,’” Tagle said.

“He is the first person surprised how he can, given his age, do so much. In a sense, that explains clearly (how and why) he does his service for the Church,” Lombardi said.

Follow Thomas D. Williams on Twitter @tdwilliamsrome.