[duanloves]

Assuming that you scouted that the zerg is going infestors, start to harass the mineral line with your unit of choice. I would go warp prisms and warp in zealots or dts and try to do as much damage as possible. During that tech to HT and research storm.
That would force the zerg to be turtle and remake economy or to push out. Make sure you have an obs to see army movement.
In the big engagements seperate your HT from your main army and try using the range advantage to feedback infestors. It would generally kill the infestors straight.
If the zerg has a really high infestor count it would be more cost effective and quicker to storm infestors as they clump up then to feedback by one unless you are have insane feedback micro.
Go watch vods of dreamhack HuK vs Moon and how he handles ling infestor ultras