I would say it's normal for them to act a little under the weather, since mine do, but some are affected more than others. In my experience, medication does not really help, and the virus usually runs its course in a month or so. So I never do anything about it (unless it has affected the eyes), and it eventually goes away, and then the birds are immune. I don't know if secondary infections are possible, but I will mention that the birds I originally vaccinated against it got the full-blown disease later on anyways, so that's why I don't vaccinate. As for separating the sick ones, I don't think it would make much of a difference (unless the sick ones are getting picked on), since all your birds have been exposed to the fowl pox if even one bird has it, so you can probably expect your other birds to get it (although some seem to be immune or just hardy, so they don't get it). I am just speaking from experience, but I don't think it's something you should worry about. It's true that birds can die from fowl pox, but it's also true that children can die from chicken pox, and it's very rare and would have to be very severe to be life-threatening.