Widespread occurrence of a disease (8)
I believe the answer is:
epidemic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">widespread occurrence of a disease</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for epidemic that I've seen before include "health emergency" , "eruption" , "Widespread outbreak of an infectious disease" , "Prevailing" , "Widespread infection" .)