Sasuke would probably be in denial at first. He has always seen Naruto as a rival and a strong force. But once he accepted the reality, he would do whatever it takes to help, maybe even going on dangerous missions to find a cure. He would also feel a sense of guilt if he had ever underestimated Naruto because of the disease.
Harry would be extremely worried and determined to find a cure. He'd probably search every corner of the library and consult all his wizarding contacts. Ron, on the other hand, would be initially in shock but then would try to cheer her up with his clumsy humor.
Naruto would likely experience a range of emotions. He could be in denial at first, not wanting to believe that this disease could end his life. Then there would be sadness, thinking about all the things he might not be able to achieve. He'd also feel guilty for making his friends worry. And there would be moments of anger, directed at the unfairness of getting such a disease.
Well, it would surely slow him down. He might not be able to take on as many dangerous missions. His physical strength could be diminished, and he'd have to rely more on his strategic thinking. Also, his friends would probably be overly protective, which could create some tensions within the team.
Ruby might be a bit shocked at first, but then she'd be really intrigued. She's always open to new ideas and would probably try to find ways to apply what she reads to her own life in Remnant. Maybe she'd start thinking about how to better protect her friends from potential disease threats.
Shikamaru would likely find the whole situation tiresome. He might think it's a bother and wonder why Wendy has to have a crush on Naruto. Sasuke would probably be indifferent at first. He has his own goals and isn't really interested in Naruto's love life. However, if Wendy's crush starts to interfere with Naruto's progress in some way, Sasuke might step in and tell her to back off. Naruto's friends like Choji and Kiba would probably tease Naruto about Wendy having a crush on him, which could make Naruto a bit embarrassed.
Sakura would be very concerned. She has known Naruto for a long time and has seen his growth. She would probably try to be there for him, using her medical skills to make sure he's physically okay and offering emotional support. For instance, she might sit by his side and talk to him about his feelings, just like she did when they were younger.
Naruto's normally upbeat and energetic personality might become more subdued. He'd have to face the reality of his own mortality, which could make him more cautious and less impulsive.
Sakura might be initially shocked but then she would be determined to help Naruto find out what's wrong. She could start researching in the hospital's library for any medical or historical records related to such a phenomenon.
Well, in some Naruto fanfictions, a disease could be used to create a really unique plot. For instance, if a disease was created that specifically targeted the Kekkei Genkai holders. It would force the different clans to work together to find a cure. This would be a huge shift from the usual clan rivalries. Maybe the disease was created by a rogue ninja who wanted to wipe out all those with special bloodlines to gain power for himself.
Sasuke might be a bit taken aback at first. He's used to Naruto's normal look, and this change could make Sasuke see Naruto in a new light, perhaps as a more formidable opponent or an even stranger ally. Sakura would probably be shocked too. She might tease Naruto about it at first but then grow to accept it. Kakashi would likely raise an eyebrow and make a comment about how Naruto is always full of surprises with his new look.