plague has 15 distinct synonyms spread across 5 of its 7 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
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
a swarm of insects that attack plants
“a plague of grasshoppers”
any large scale calamity (especially when thought to be sent by God)
an annoyance
“those children are a damn plague”
Synonyms, antonyms and the broader and narrower relations for plague come from Princeton University’s WordNet. Full sources and licences.