Of all the causes for lameness in the horse, I believe the most common is infection in the foot. We sometimes don't categorize foot infections in horses with other lameness such as navicular disease or ringbone, because of its transient nature and usually complete recovery, but it is still the most common cause of a lame horse.
In Central California, foot infections, often referred to as "abscesses," are a seasonal disease in horses. I see a spike in cases during the rainy or wet season, which implies the etiology of the problem. Moisture works its way up through small defects or the tubules that are at the junction between the sole and hoof wall. Moist anaerobic (i.e., no air) conditions are an incubator for barnyard bacteria to proliferate and infect the sensitive lamina or soft tissue that interfaces the hoof capsule and coffin bone.
The sensitive lamina are rich in sensory nerve endings, just like what's under our finger- or toenails, so inflammation in this area causes profound pain and symptoms. This condition can evolve overnight, so you may go out in the morning to see your horse almost non-weight-bearing on one leg, when he was perfectly normal the day before.
Because this condition is the result of acute inflammation from infections in the foot, lower leg edema may be present. Oftentimes, the severity of the lameness and the swelling in the lower leg are suggestive of severe trauma. As a veterinarian, the first thing I do in an exam is evaluate the intensity of the pulse in the posterior digital arteries. One can feel a "throb" or rise in intensity of the pulse. The next thing I do is go over the foot with hoof testers and let the horse tell me where the infection is located.
Then I start exploring with a hoof knife for any black lines or suspicious looking spots to follow to possibly open an abscess. Early in this condition there may be no pus pocket to open, so I treat it like any other early abscess. Apply heat by soaking the foot in hot water and Epsom salts. I suggest soaking the foot for 10 minutes twice daily. The water should be as hot as you can comfortably hold your hand in, and add a cup of Epsom salts to the gallon or two of water it takes to immerse the entire foot when using a small pail.
This condition has traditionally been referred to as a "gravel," implying that a small stone or piece of gravel somehow worked its way up through the sole and popped out at the hairline where a lot of these infections wind up draining. But in 44 years of looking at horses with this condition, I have never seen a piece of gravel involved.