The plague

The day every mammal with a Y chromosome died, except Yorick and Ampersand.

via Wikipedia: Holocene extinction · see source

In Y: The Last Man, on July 17, 2002, every living mammal with a Y chromosome, including embryos and sperm, dies at the same moment. The only exceptions are Yorick Brown, a young amateur escape artist, and his Capuchin monkey Ampersand; the comic also names Doctor Matsumori among the survivors. The source of the plague is never fully explained, and the protagonists weigh three explanations for it. Birds, reptiles, amphibians, fish and invertebrates are unaffected because they mostly use different sex chromosomes.

Quick Facts

Date

July 17, 2002

Survivors

Yorick Brown
Ampersand
Doctor Matsumori

Cause

Never fully explained

Facts from the source article.

Background

Doctor Allison Mann, a geneticist, sets out to discover the cause of the plague and why Yorick survived; Yorick's sister Hero joins the Daughters of the Amazon after it. The television adaptation keeps the premise of a cataclysm that kills every mammal with a Y chromosome but for Yorick and Ampersand. Showrunner Eliza Clark stressed that it is Yorick's Y chromosome, not his maleness, that sets him apart: the series specifies that trans women with Y chromosomes died, as did women with androgen insensitivity who did not know they carried one, while trans men survived.