It's caused by Sarcocystis neurona, a parasite that affects the central nervous system. Most horses carry the parasite without problems, and it has a secondary host - generally an opossum, a cat or a raccoon.
The infection causes lesions in the brain or central nervous system as well as inflammation.
Horses infected by EPM can suffer permanent damage and it can be fatal if untreated. Treatment is a relatively recent thing and consists of antiprotozal drugs to reduce the parasitic load and anti-inflammatory drugs to control inflammation and reduce damage.
The worst thing about EPM is the long list of symptoms, many of which are common with other neurological diseases. The first symptom is usually facial paralysis, an ear drooping, or difficulty chewing and controlling the tongue.
Other symptoms include uncoordinated movement of the rear feet, generally worse on one side, intermittent lameness that may move from one leg to another, hind end weakness, and problems balancing when a hoof is lifted. Muscle atrophy may also be seen, the affected horse might lean on something for balance, not stand square, and drag a hoof when turning. The back may be sore, the horse might sweat up for no reason or carry his tail crooked. Changes in vision, tilting the head to one side, and behavioral changes are also observed - and any of these may become permanent. Horses with odd personality traits, rear end problems that aren't quite bad enough to make them lame, clumsiness and even anxiety may be animals that, at some point in their life, had EPM...but there are all sorts of other things that can cause it.
(In other words, consider using this as a simple explanation for equine neurological problems, but bear in mind it's most commonly carried by opossums and is rare outside areas they exist in).
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