Catechism of the Catholic Church §1008 — excerpt
Death is a consequence of sin. the Church's Magisterium, as authentic interpreter of the affirmations of Scripture and Tradition, teaches that death entered the world on account of man's sin.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: DS 1511 · GS 18 # 2