I'm not sure how to ICly affect characters who never get sick for supernatural or magical reasons, but for characters who never get sick because they're physically extremely resistant to disease, I might have an idea. Marina brings together a lot of people from different worlds/realities/species, so what if a few of them are carrying corona viruses (aka the type of virus that usually causes colds) that normally don't cause illness because the native population has built up an immunity to them over time. However, these viruses could cause illness in people who aren't from those locales because those people have no natural immunity to that particular strain. Also, since cold viruses tend to mutate pretty quickly and thus outwit immune responses, that could help the virus spread across a large segment of the population.