Highly infectious (8)
I believe the answer is:
virulent
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">highly infectious</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 virulent that I've seen before include "Highly infectious (disease)" , "(Of disease) extremely severe" , "[OPPOSITE OF] Harmless" , "Venomous" , "Violent or poisonous from a liver nut" .)