Like the flu
I believe the answer is:
viral
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">like the flu</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for viral that I've seen before include "as an internet sensation" , "Like some infections" , "Like many diseases" , "Pertaining to an ultramicroscopic infection" , "Like pathological agents" .)