This details common illnesses that may effect horses.

Regular vaccination and preventative health care is just as necessary for horses as for any other pets to keep them healthy. Health examinations by a veterinarian should be given regularly, even if your horse seems healthy. Illnesses or diseases often causes minor changes in your horse's physical appearance or blood work that needs to be observed by a veterinarian.

Common illnesses, like flu, can be prevented by immunization and therefore all horses should receive a flu vaccine at least twice a year. Horses that compete in events should be revaccinated four times per year, as this vaccine does not protect horses for long periods of time.

http://www.horse-stall.net/horse-health/preventative-horse-health-care.htm