A farmer grew upset when fighting became contagious? (4,7)

I believe the answer is:

germ warfare

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'fighting became contagious?' is the definition.
I can't judge whether this defines the answer.

'a farmer grew upset' is the wordplay.
'upset' indicates an anagram.
'a'+'farmer'+'grew'='afarmergrew'
'afarmergrew' with letters rearranged gives 'GERM WARFARE'.

'when' is the link.

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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