Then it will show up as disease. Parasitism may be present, but not causing any problems, that is, until the bird breaks with a disease. Parasites live off the host either by feeding directly on host tissue or using the nutrients taken in by the host. In either case the bird needs to be constantly working to stay even. When an additional insult occurs, like the invasion of bacteria, the bird may not have enough left to fight it off.
Antibiotics work by either stopping the growth or directly killing the bacteria.
Different antibiotics also have particular bacteria that they work against. When infectious disease is present we must determine the type and location of the bacteria to determine the choice of treatment. This requires testing in a diagnostic lab. Using the wrong antibiotic may actually make the disease problem worse. Indiscriminate use of antibiotics can affect the good bacteria that help keep everything in balance.