The older horses and some younger horses, particularly on racetracks, develop an inflammatory airway disease which we believe is non infectious. I don't think that anyone knows the role played by infection in such horses. Horses in the northern hemisphere are stabled for a good proportion of their lives and it's strongly believed that the environment in which the horse is kept plays a very large part in the problem of low grade respiratory disease. At present investigators can't agree on how to define respiratory disease. If we used this measurement, we would classify a huge number of our horses as having chronic airway disease.
Trainers generally say that respiratory disease is second only to lameness as a problem.
I don't know of any good data on the true prevalence of certain types of respiratory disease. No, respiratory disease in almost all species, even humans, tends to be a major cause of lost performance. Respiratory diseases rarely kill any mammal but they reduce productivity by days lost from work and from training. With low grade respiratory disease there is usually a cough and maybe poor performance. The diagnosis often is one of exclusion and it is necessary to examine the horse for lameness or heart problems etc.