The spelling is diarrhea. It is certainly not a term that gets used in common every day discussions, although it is used when appropriate. It's one of those scenarios wherein native speakers use it in timely ways and, if misused, can sound like a train wreck. Medical practitioners use it to describe "loose, runny poop." (ewww). Native speakers use it if they have it to describe the condition to others, but discretely not like a public service announcement. More often, native speakers who a very familiar with each other might use a slang terms such as "I've got the shits" or "I've got the runs." But once again, native speakers know when to use the terms. I wouldn't recommend it until you get a sense of when to use it appropriately. It would be more discreet to say "If I eat it, it might upset my stomach or make me sick."

-I'm afraid that if I eat it, I'll get diarrhea. There shouldn't be an "a" in that sentence. (That's not a countable noun) Also, as Terry said, you most likely would say "I'm afraid that if I eat it, I'll get sick." and you leave the exact nature of the sickness to the hearer's imagination.

Just to clarify the difference in the two answers you have had, your spelling of diarrhoea is correct in the UK and in many other countries but in the US they drop the O. It looks like Canada is the same, judging by Terry's reply.

I'm afraid *if* I eat it I'll get a diarrhoea. Don't eat *it*! It *doesn't look* fresh. You may get diarrhoea. Susun can't go out because she has diarrhoea. <-- correct.

And for everyone's amusement: https://www.youtube.com/watch?v=l-4WbjV1Jmo