nasty
flu, stomach, tummy

have
catch, come down with, get, pick up

go around, go round (esp. BrE)
A stomach bug has been going around at school.

strike sb down (BrE)

with a/the bug
He's off work with a flu bug.

be bitten by, catch, get
She's been bitten by the travel bug.

bite (sb)

minor
annoying
computer, software

discover, find
fix

bug in
a bug in the software