As Terran
Make:
Ghost
Liberator
RavenGhosts are the main answer. Scan before the engagement.
Open in the counter tool with Infestor tagged ->As Protoss
Make:
High Templar
Disruptor
ObserverFeedback or burst removes Infestors before they lock the army.
Open in the counter tool with Infestor tagged ->As Zerg
Make:
Overseer
Lurker
ViperDetection and spellcaster energy decide Infestor mirrors.
Open in the counter tool with Infestor tagged ->
Zergling
Baneling
Roach
Ravager
Queen
Hydralisk
Mutalisk
Corruptor
Overlord