Spreading fast online (5)
I believe the answer is:
viral
'spreading fast online' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for viral that I've seen before include "Of organisms causing 'flu, etc" , "Spreading rapidly" , "Like some infections" , "Like the common cold" , "Relating to a pathogenic agent of disease" .)