Half a dozen Russians contract an infectious disease (5)
I believe the answer is:
'disease' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for virus that I've seen before include "Infective particle" , "computer problem" , "Electronic threat" , "Hidden destructive code in a computer program, or cause of illness" , "Cause of infection (in people and computers)" .)