Let's examine the Infested Terran for a second (on an unrelated not, an Infestor can use fungal growth to do far more economic damage than the Infested Terran).
- Speed: 0.9375 (that's the same speed as a Queen off creep)
- DPS: 9.3 (which is actually more than a Marine)
- Hit Points: 50
- Range: 5
So they're slow, and flimsy, but do decent dps.
Depending on your strategy there are a couple solutions to this problem:
- High Templar -> Between Feedback (for the Infestors) and Psi Storm for the Infested Terran you're good to go. Because Infested Terran are so slow, Psi Storm will destroy them
- Colossus -> Always a good choice and with the Range upgrade you actually out Range them by enough they won't ever get near, though mind control can turn this against you in a normal battle
- Phoenix -> Not the best choice as they tend to die to Infested Terran, but grabbing an Infestor is a very good tactic and the Phoenix will make short work of them.
- Observer -> Strangely enough this unit alone will deal with most of your problems. As long as you have a nice choke, the Observer will spot the flimsy Infestors (80 hp).
Now assuming you don't have any of those things here are a few things to keep in mind: Chargelots tear Infested Terran apart, but even smarter, they run away faster than Infested Terran can follow. Little know fact is that the Charge upgrade increases Zealot normal movement speed. Infested Terran have very short life span (30 seconds) so if you can just stay away from them you should win easy. Another important point is Infested Terran attack very quickly so Guardian Shield is your friend (not to mention Force Field + wait).
When faced against Infestors you really need to invest in detectors. Though you may have had a run in with Infested Terran, the real scary weapon is the Fungal Growth. In addition to the 4.5 aoe dps, Fungal Growth locks down your units leaving them at the mercy of a Zergling attack or long range Hydralisk force (range 6).
If you spy a Zerg going Infestors, its always good to build an Observer or 2. One for your force and one to watch your base (Fungal Growth makes short work of Probes).
Edit: its worth noting that Neural Parasite (mind control) is an upgrade to Infestors, and that many Zerg skip it, or don't get it till late game. Given the power of Fungal Growth, and the short duration of Neural Parasite, its easy to see why.