Infection (5)
I believe the answer is:
virus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">infection</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for virus that I've seen before include "Bodily or computer infection" , "Harmful computer code" , "Malicious code" , "Microscopic entity capable of reproducing within cells" , "Organism causing disease, or malicious program in computer" .)