Yes, DIARRHEA is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, A 1, R 1, R 1, H 4, E 1, A 1.

Is it a valid word?

DIARRHEA is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

frequent and watery bowel movements; can be a symptom of infection or food poisoning or colitis or a gastrointestinal tumor

Also: looseness of the bowels.