Collocations for flu - noun

mild
bad
gastric (BrE), stomach (AmE)
avian, bird
pandemic

bout (esp. BrE), case, dose
She's had a nasty dose of flu. (BrE)
She's had a nasty dose of the flu. (AmE)

strain

be in bed with (esp. BrE), have, suffer from
catch, come down with, contract, get, go down with (BrE)

bug, virus
epidemic, outbreak, pandemic
symptoms
victim
vaccine
jab (BrE), shot (AmE), vaccination
strain
season (= time when a lot of people suffer from flu)

a bout with the flu (AmE)
She was off work for four days because of a bout with the flu.