The Hashiras would likely show concern. Some, like Shinobu, would immediately start looking into possible medical solutions. Giyuu might be more reserved in his worry but would still keep a close eye on Tanjiro. The other demon slayers would also be affected. They look up to Tanjiro, so his sickness would lower the morale of the group. They would try to support each other while waiting for Tanjiro to recover.
Sure, Tanjiro masters Sun Breathing in the manga. It's a turning point that showcases his determination and progress. It also ties into the broader story and his quest for justice.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.
They might be shocked at first. In the fanfiction, there could be a lot of dark and unexpected plot twists. For example, if their relationships are depicted in a different way or if there are new threats they have to face. They'd probably start by discussing among themselves what they just read, trying to figure out if it has any real meaning or if it's just a wild interpretation of their world.
Well, Naruto would probably be really confused at first. He comes from a world of ninjas and chakra, and The Last of Us is a post - apocalyptic world full of infected and survival struggles. But being Naruto, he'd also be curious. He might be shocked by the violence and the desolate nature of the world in the fanfiction. He'd likely try to find some way to bring hope to the characters, like he always does in his own world.