Well, sort of.
Portal is about two kids getting trapped in an alternative universe. The universe they ended up in was like ours, but a couple hundred years behind us technologically. I set it up that way because I wanted a bit of A Connecticut Yankee in King Arthur’s Court feeling to the novel — the kids had to figure out how to make do without modern inventions — no cars, no planes, no electricity . . . But then they use their middle-school knowledge to actually make a difference.
But why was this world a couple hundred years behind ours? I didn’t exactly have to explain this; my premise was that anyt event could split off another universe, as in Everett’s many-worlds theory. (Did I ever mention that I once saw his son perform at the Somerville Theater? I digress, however.) But I thought it would be an interesting plot point. So early on in the novel I had the kids figure it out: native Americans had a horrible disease (I called it drikana) that the first European explorers brought back with them. And it devastated Europe, the way that, in our world, smallpox devastated the Americas. The story of Guns, Germs, and Steel in reverse. Eventually Europeans built up immunity to the disease, but in the meantime the Enlightenment and the Industrial Revolution were delayed, and so the world the kids found themselves living in was still primitive compared to ours.
And of course the kids had no immunity to drikana. One of the kids comes down with it, and both end up quarantined, wondering if they will survive. Much drama and pathos ensues.
Having used this plot element in Portal, I didn’t trot it out again in its sequels, Terra and Home. But this nagged at me a bit — if I wanted this bit of realism in novels (that otherwise weren’t realistic in the slightest), I really should have had the threat of disease be a pervasive concern whenever you traveled to another world. If I were writing these novels nowadays, this threat would have loomed a lot larger, I’m sure.