Pope Francis encouraged Catholics to show affection for the elderly during his weekly prayer on Wednesday.
The pontiff said that mature individuals should be treated with respect and admiration, and not cast aside as in some societies.
"It's a mortal sin to discard our elderly," he told the crowd in St. Peter's Square. "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."
Francis, who is 78 years old, warned against ageism, and said that old age is treated like "an illness to be kept at arm's length".
"If we do not learn to look after and to respect our elderly, we will be treated in the same way," he insisted.
"A society where the elderly are discarded carries within it the virus of death."
Pope Francis has consistently called for unity against injustice, and has made the plight of underprivileged populations, exploited workers, and victims of modern slavery central causes for the Vatican.
Pope Francis used the World Day of Peace service in January as an opportunity to condemn slavery and human trafficking across the globe, and the theme of the New Year's Mass at St. Peter's Basilica was "No Longer Slaves, but Brothers and Sisters."
The call for unity against injustice echoed remarks the Pope made in December regarding sweatshops. He urged consumers to think twice before purchasing low-cost goods and material that may have been manufactured under oppressive means.
Pope Francis also came to the defense of underprivileged populations and exploited workers during a New Year's Eve Vespers service. He criticised Rome's politicians accused of stealing funds meant to help migrant workers, and called for a "spiritual and moral renewal" in the ancient city.