Yes, PARASITES is a Scrabble word!

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

Is it a valid word?

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

an animal or plant that lives in or on a host (another animal or plant); it obtains nourishment from the host without benefiting or killing the host
a follower who hangs around a host (without benefit to the host) in hope of gain or advantage