Both endemic and epidemic are used as either a noun or an adjective. As a noun, endemic is partially defined by Merriam-Webster as "characteristic of or prevalent in a particular field, area or environment." As a noun, it defines epidemic as "an outbreak or product of sudden rapid spread, growth or development."
As an adjective, the dictionary defines endemic as "characteristic of or .
prevalent in a particular field, area or environment." As an adjective, it defines epidemic as "excessively prevalent or contagious."
As nouns, these two words have completely different meanings. As adjectives, they are similar, but non-synonymous. In the sentence "There was epidemic laughter when Jenny entered the meeting with her shirt on backwards," the word endemic could not be substituted.
More Reference Links: http://www.merriam-webster.com/dictionary/endemic http://www.merriam-webster.com/dictionary/epidemic
