As a writer of fantasy and science fiction I can use my imagination to create new unique things in every part of the worlds I create. My characters don't have to human. They don't have to need oxygen to breathe or food as we know it to live. But readers love stories where they can identify with or at least understand the hearts and minds of the protagonists. If the reader struggles to understand the world I've created because it's so different from the real world, they might put the book down and never pick up anything I've written again. There must be a balance.
I've never written historical fiction though I love reading it. I too big a coward to tackle historical fiction. Readers will spot those mistakes and let you know or worse let the whole world know in a review. But since the epic fantasy I write is set in a medieval setting, I try to insert some realism into the cultures I create.
A book I keep on my desk, What Kings Ate and Wizards Drank by Krista D. Ball, covers loads of interesting facts. For example, how long does it take to make a stew over an open fire? Two hours to get that meat tender. What vegetables and roots could travelers find in the woods? Does the hero carry a thick, metal cooking pot and the big knife needed to field dress game? Great answers in this book?
Getting enough to eat was a real concern during medieval times and malnutrition often made people vulnerable to other illnesses and diseases. Of course there was the scary bit one, the plague, that wiped out a third of Europe. And it's still around today though not nearly as frightening. Read about modern outbreaks. Of course in a fantasy novel magic is always an option for health woes but I can add some depth to my society by also including real methods that might be used in a medieval society. The History Learning Site has lots of interesting information on medieval life. Another resource I use is a book of herbal remedies, many that are served in teas. Doesn't every medieval book need to serve up a little tea now and then.
Did you ever read a historical novel and spot a glaring error? How about a fantasy novel or science fiction novel where you just didn't buy into a piece of the story? Ever read a novel like I did last week that was so confusing in the world building I couldn't get interested? Had you heard about the recent cases of plague in Colorado?