Bacterial pneumonia is a serious respiratory infection in horses. To prevent it, make sure your barn is well ventilated and clean. Give your horse plenty of time to recover if he has just undergone strenuous exercise – his immune system will be down.
Whenever possible, don't tie your horse's head up when shipping, and avoid hay bags. Have any horse with a cough or fever evaluated by your veterinarian; most horses will have a viral infection, but a few will go on to get pneumonia.
For more information, please read the article Bacterial Pneumonia in the Horse.