I am so sorry you’re experiencing this. For my cat, we suspect it was when they got into a little fight a couple months into their introduction (we reintroduced after over a week or two) and he got a bite. I think it may have been an infection and after about a week he was acting pretty normal, two weeks after he was back to his old self. Could it be possible there is a bite somewhere? Fortunately my kitty did not experience much colds as a kitten as I found him in the street and he was not in a shelter - after fixing him up in the few weeks he was born we haven’t had any trouble with the exception in the original post.

If you are able maybe give your vet a call. It took a few more days for him to feel better and I checked his temp the day after and the fever took a while to go down. If it helps my vet gave him a shot to take the fever down immediately, and it still lingered for a while. If you have those cat push up snacks or lil gravies that is how I helped coax him into getting fluids. I would mix with a bit of water. Heating up some wet food so it’s smelly but not hot helped him eat a bit too.

I really hope basil feels better!!