After showing astonishing resilience throughout a fast-paced 12 days in four countries in Asia and Oceania earlier this month, Francis, 87, cancelled his audiences on Monday due to a "mild flu".
The Vatican said the pope, who had part of one lung removed in his youth, needed to rest before he resumed his travels.
The heart of the Argentine pontiff's 46th trip abroad will be addressing priest paedophilia in Belgium, tarred by decades of scandals and cover-ups.
The confessions of a disgraced bishop 14 years ago prompted thousands of victims to come forward.
During his stay, Francis will meet privately with 15 victims of clerical sexual assault, arranged by the country's bishops' conference after a hard-hitting documentary last year put the scandal back on the front pages.
In it, victims revealed secrets often buried for decades. They denounced Church efforts to protect aggressors and difficulties in getting justice.
"Yes, there has been a culture of secrecy and silence in the past within the Church, which has made each of these tragedies even harder to bear," Luc Terlinden, head of the Conference of Belgian bishops, said ahead of the trip.
It is "necessary for people to speak out", he said, adding that the meeting with Francis was "an important symbolic step".
In an open letter published by Le Soir newspaper, victims demanded the pope address paedophilia and the thorny issue of priestly celibacy.
- Forced adoptions -
Francis may also speak about the forced adoptions scandal, by which institutions run by nuns took in underage girls and pregnant unmarried women until the late 1980s and gave their children up for adoption.
Belgium's HLN news site estimates that up to 30,000 children were taken from their mothers in Belgium between 1945 and the 1980s.
Bishops in Belgium apologised in 2023 and requested an independent investigation after fresh testimonies emerged from women and people claiming to have been "sold" by the Catholic Church to their adoptive family.
The trip -- officially organised to mark the 600th anniversary of the Catholic University of Louvain -- will begin with an eight-hour visit to Luxembourg, where Francis is expected Thursday at 10:00am (0800 GMT), meeting the Grand Duke at Notre-Dame Cathedral.
Francis will then begin a three-day visit to Brussels Thursday evening, during which he will be received by King Philippe at the Palace of Laeken and will meet the prime minister and the clergy at the vast Basilica of the Sacred Heart.
Francis will hold two meetings at the Catholic University of Louvain in Flanders (UCLeuven), one with academics on Friday and a second with students on Saturday at Louvain-la-Neuve in Wallonia (UCLouvain), notably on climate issues.
The pontiff has made protecting the environment one of the major themes of his papacy, and has repeatedly lambasted man-made global warming, calling for concrete political changes to stop the planet's climate spinning out of control.
The pope will preside Sunday at a large open-air mass at the King Baudouin stadium in Brussels, where 35,000 people are expected.
The last papal visit to Brussels was in 1995, when John Paul II attended the beatification of Saint Damien, who dedicated his life to lepers.
Nearly 65 percent of the Belgian population is Christian, including 58 percent who are Catholic, according to figures from UCLouvain.
But their numbers are on the wane, reflecting a decline in Europe as a whole.
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