Yes, DIPHTHERIAS is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, P 3, H 4, T 1, H 4, E 1, R 1, I 1, A 1, S 1.

Is it a valid word?

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

acute contagious infection caused by the bacterium Corynebacterium diphtheriae; marked by the formation of a false membrane in the throat and other air passages causing difficulty in breathing