The main explanation given for Yorick’s survival is actually related to Ampersand and his … monkey feces. It seems silly, but basically, Ampersand isn’t just any capuchin. He’s been experimented on in a way that leaves him immune to the plague. And because Yorick cares for and cleans up Ampersand’s feces, Yorick also gained immunity from the Y-chromosome destroying plague in a lucky twist.
While this is the most accepted explanation for Yorick and Ampersand’s survival, it’s not fully confirmed. This makes sense because Brian K. Vaughan, the creator of the Y: The Last Man comics, offers several theories of the plague’s origin. So, Yorick and Ampersand’s survival is dependent on whichever theory we believe.
What we’ll call “the feces explanation” ties directly into a theory about how the plague came to be. The geneticist, Dr. Mann, who plays a major role throughout the series, believes that her cloning research wiped out Y-chromosome-bearing mammals as a form of evolution because sexual reproduction would become obsolete.
In this theory, Ampersand was given a concoction that was supposed to sabotage Dr. Mann’s work, but he was accidentally shipped to Yorick, saving them both.