Crossword Genius

Fighting that's contagious? (4,7)

Ross

I believe the answer is:

germ warfare

I'm a little stuck... Click here to teach me more about this clue!

'fighting' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe there's an association between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Another definition for germ warfare that I've seen is " threatening use of biology".)

I've seen this clue in The Daily Express.
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