Question: Are there any diseases like the one described in the book "The Enemy" Where all adults catch a disease which kills most of them, but turns the rest into zombies, which become less ill the more kids they eat, and is it at all feasible?

I haven’t read the book you’re talking about, but I looked it up online and looked at the plot description. I’m happy to say that no, there aren’t any diseases like that (that I’m aware of, anyways!). I mentioned why it’s unlikely to ever happen with a virus in this question, and the same things I said there apply to most other parasites like bacteria and fungi that infect hosts.