Hi, I hope it's ok to ask such questions here (I've waited till Tuesday, since it seems to be your "casual posts" day).

I have a worlbuidling project that I'm working on. It's going to be focused on a city under a quarantine that has lasted for over a thousand years. Since it's a fantasy story, I can take some liberties with what is possible and what isn't, but I'd like it to be believable, so I want to ask you some questions:

1. My assumption is that long time ago, there was a plague that caused the city to close. People who weren't immune have died, but the city can't open, because all of the people in it, while immune, are still carriers. Is that possible?

2. Part of the story is working under the assumption that most children are not immune to the virus, causing most pregnancies to fail, or dying in infancy. Is that possible, or would a child of an immune couple also be immune?

3. Finally, since the city population is dwindling, it depends on immigration for its existence. Question is - what level of technology would be required to test whether potential immigrants are immune? Is there a standard procedure for any type of illness?

Hope I can get some insight here and learn something new, thanks!