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.
In a fanfiction about Tanjiro getting sick, it could be that he overexerts himself during a mission. His body gives in and he falls ill. His comrades take him to a safe house. While he is sick, he reflects on his journey so far, his losses and his goals. Meanwhile, the others keep a lookout for any danger and also try to nurse him back to health. They might face shortages of supplies and have to make difficult decisions.
In addition to his family and friends, the medical ninja in the village would be involved. They would be the ones to accurately diagnose Boruto's condition. Shizune, for example, could come and check on him, using her vast medical knowledge. And the other young medical ninjas might learn from this case, as Boruto's illness could be something new or unusual, so they would be eager to assist and gain experience while taking care of him.
Seth could be the one to help her. As her brother, he would be very concerned and might use his own knowledge of Fablehaven and its secrets to try and find a cure. He could also enlist the help of some of the friendly magical creatures they've befriended, like the fairies or the satyrs.
His teammates, Sasuke and Sakura, would surely be involved. Sasuke, although he may not show it as much, still cares about Naruto. He would keep an eye on Naruto's condition, and if there was any sign of danger, he would be ready to protect him. Sakura would be more hands - on with the medical care, checking his temperature, giving him medicine, and making sure he rests properly.
Sango could also take care of Kagome. As a strong and kind - hearted woman, she would know how to nurse Kagome back to health. She might use her knowledge of herbs and remedies to make Kagome feel better. Plus, she would offer emotional support to Kagome during her illness.
In the 'Boruto Gets Sick' fanfiction, Boruto might experience various symptoms like a high fever, weakness, and coughing. His friends and family would likely be worried about him. They would probably take care of him, bring him medicine and try to make him comfortable.
In the 'tohru gets sick fanfiction', perhaps her friends would be very worried about her. They might take turns to take care of her, bringing her warm soup and making sure she takes her medicine on time.