It is important to realize that antibiotic treatments are used only to eliminate or shorten the duration of existing infections, or to prevent secondary bacterial infections. These infections may reduce performance or threaten the life of the animal if treatment is not provided in a timely fashion. However, the animal's immune system is the real key to recovery. There are many causes of disease and numerous predisposing factors. A predisposing factor is something that makes the animal more susceptible to infection.
Many diseases have a single cause agent, such as a specific virus or bacteria.
Some diseases may have specific predisposing causes while others are due to a microorganism or virus, plus increased animal susceptibility. Treatment selection and procedures for some diseases frequently can be quite simple. In the case of mastitis, respiratory disease and others, however, the story may be quite different. Diseases due to infectious bacteria are common, and antibiotic treatment is helpful in many situations. There are literally hundreds of agents and causes of infections.