outbreak vs occurrence

outbreak and occurrence both are nouns.

Word	Noun	Adjective	Verb	Adverb
outbreak	Yes	No	No	No
occurrence	Yes	No	No	No

As nouns, occurrence is a hypernym of outbreak; that is, occurrence is a word with a broader meaning than outbreak:

outbreak: a sudden violent spontaneous occurrence (usually of some undesirable condition)
occurrence: an event that happens

Other hypernyms of outbreak include happening, natural event, occurrent.

outbreak (noun)	occurrence (noun)
a sudden violent spontaneous occurrence (usually of some undesirable condition)	an event that happens
	an instance of something occurring