Joel's medical condition in the HBO show is actually canon thanks to The Last of Us Part 1 Remake

The Last of Us game addresses the issue from episode 6 in a very subtle way

HBO's Live-Action 'The Last Of Us' Series Confirms Race-Swap For

Tess Servopoulos, The Last of Us Wiki

The Last of Us HBO: Joel Was Right Mostly - IGN

Everything that sucks! And some things that don't.

Last of Us season 1 ending: Did Joel do the right thing? - Polygon

The Last Of Us' Restarts The Great 'Was Joel Right?' Debate

The Last of Us, The Last of Us Wiki

Last of Us season 1 ending: Did Joel do the right thing? - Polygon

A 'The Last of Us' Fan Theory Figured Out Shocking Season 1

What is your problem with The Last of Us Part 2? - Quora

The Last of Us Part 1 Confirms Concerning Details About Joel's Health

HBO's Live-Action 'The Last Of Us' Series Confirms Race-Swap For