Pope Francis urged his followers at his weekly prayer on Wednesday to show affection and respect for the elderly, saying they must not be treated as "aliens".
"It's a mortal sin to discard our elderly," said the pope, seen by his supporters as a champion of the poor and marginalised.
"The elderly are not aliens, we are them, in a short or in a long while; we are inevitably them, although we choose not to think about it," he told 12,000 followers gathered on Saint Peter's Square.
"A society where the elderly are discarded carries within it the virus of death," said the 78-year-old Argentinian pontiff.
The pope said that while life expectancy has increased in recent decades, old age is treated like "an illness to be kept at arm's length".
He warned his followers: "If we do not learn to look after and to respect our elderly, we will be treated in the same way."
Pope Francis, wearing the mantle of a humble advocate for the poor while defending Catholic orthodoxy, rose from modest beginnings to become, in 2013, the first Latin American leader of the 1.2 billion-strong Church.