The answer is:
PANDEMIC: a disease spread over a whole country or worldwide.
Root: dem
Affixes:
pan-: prefix expressing inclusion of a continent, ethnic or religious group .
-ic: suffix that forms adjectives.
A word definition is the precise meaning of a word, especially found in a dictionary. The root of a word is its basic form, without any affixes. Affixes are morphemes connected to words -prefixes and suffixes- that change their meaning: they provide different grammatical categories or inflection.