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.
Well, it can be considered a spoiler if you haven't reached that part of the story yet. But if you're okay with knowing some key details upfront, then maybe not.
Well, through the Dance of the Fire God, Tanjiro's character develops in multiple ways. Firstly, his sense of purpose becomes even more defined. He realizes the importance of the dance not just for himself but for the whole world he's trying to protect from demons. His compassion grows as well. When using the dance, he might see the pain of the demons more clearly, which in turn makes him more empathetic. His leadership skills start to emerge too. Other demon slayers might look up to him when he showcases the power of the dance, and he has to take on more responsibilities. Also, his determination gets a huge boost. Each time he performs the dance, he is more determined to end the reign of demons.
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.
Dabao was described as a smart and kind-hearted child in " Celebrating Years." Although he was mentally handicapped, he was kind and amiable. To Fan Xian, he was a source of joy and spiritual sustenance. Only by being with him could Fan Xian truly feel the true, good, and beauty of interacting with others. Dabao was the son of Prime Minister Lin Ruofu, and his mother was the Eldest Princess. On the surface, he was Xiang Lin's son, but in reality, he was not his biological son. He was the illegitimate child of the Eldest Princess and the Qing Emperor. Like Fan Xian, he was secretly born by the Qing Emperor. Da Bao's kindness and intelligence combined to make him full of wisdom. Therefore, Da Bao was kind and smart.
Dabao was portrayed as a smart and kind child in "Celebrating Years." Although he was mentally handicapped, he was kind and amiable. Fan Xian was especially friendly to him. The two of them had a good relationship. Dabao was the son of Prime Minister Lin Ruofu. His true identity was the illegitimate son of the Qing Emperor and the Eldest Princess. Although Da Bao acted dumb on the surface, he was not really stupid. He was just a little stupid. He played an important role in the play and helped Fan Xian reveal the Qing Emperor's true colors. In the end, Da Bao died and became the leader of the ascetics in the temple, Da Hong Pao. To sum up, Da Bao showed his kindness and intelligence in "Celebrating Years."