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.
Nagisa's best friend might be really surprised at first. But then, being a true friend, would support Nagisa through this strange change. They would probably help Nagisa with things like choosing girl - appropriate clothes or dealing with any mean comments from other students.
Their other classmates like Okuda would be very sweet. She might make them some special potions or concoctions as wedding gifts, something that symbolizes good luck and a long - lasting marriage. And she would be all shy and cute about presenting it to them.
Kayano would be completely devastated. Since she had a special bond with Nagisa, she might become withdrawn and filled with rage, vowing to take revenge on whoever was responsible.
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.