Yes, PLAGUE is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, A 1, G 2, U 1, E 1.

Is it a valid word?

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

a serious (sometimes fatal) infection of rodents caused by Yersinia pestis and accidentally transmitted to humans by the bite of a flea that has bitten an infected animal
any epidemic disease with a high death rate
a swarm of insects that attack plants
any large scale calamity (especially when thought to be sent by God)
an annoyance

Not playable in Scrabble US: PESTIS, CHEVVY.