I disagree with Speed Racer. The horse either has an infection, or it doesn't. You want to treat the infection long enough to kill all of the protozoa.
Killing off the protozoa is only the first step. If the horse has irreversible neurological and nerve cell damage they will never
recover fully, thus they're not able to be cured.
Early detection and therapy may increase the chance
of sucessful treatment, but response to treatment is highly variable.
Some horses do return to their original level of function, while others remain compromised. There are also those animals who will require medication indefinitely.
So your cut and dried statement, "either the horse has an infection or it doesn't", doesn't work for EPM. It's an extremely frustrating and expensive disease to treat, and many do not recover to their pre-EPM levels.
Even the treatments and medications aren't without risks. Anemia, diarrhea, abortion, and low white blood cell counts are some of the side effects.
Hrsecrzie, I'm sorry about your gelding. EPM is a very serious condition, and I hope your boy can return to his normal work. Your vet is the one who knows the horse, so if he says you can work him lightly, go ahead.
Regardless of the advice you get from a bunch of strangers over the internet, your vet is still your best source of information and direction.