How far can you go before the improbable becomes ridiculous? Where is the line drawn between fiction and science fiction?
For example: Imagine a YA novel where the characters live an everyday life. They go to school, they have friends, there is romance, there is sorrow… and then there is a cataclysmic event like a giant asteroid hitting the planet and everything is thrown into disorder. Okay, at this point we are still in fiction, but what if after this asteroid hit people were affected with some sort of disease that caused them to selectively develop gills or an extra limb or disintegrate into a pile of ashes or something within days or even hours or minutes.
Have we moved to science fiction and/or fantasy? Can it still be plain ol’ fiction? Do people say that could never happen in such a short period of time, this is ridiculous, and throw the book across the room? How much of a plausible exclamation needs to be given? Can you attribute it to some unknown chemical in the asteroid and leave it at that? Also, Is the zombie genre considered fiction or science fiction?
This is just an example and not something I am writing (except for the zombie part), but what I am writing has the potential to turn into something unbelievable or something pretty cool. I am just trying to figure out where that line is.