Many bacteria are normally present in the body and some of them are helpful, for example they break down food in the digestive tract so that the body can use it. However, others are harmful to the body and cause disease.
Bacterial disease is treated with medicines called antibiotics, for example penicillins.
You should always follow your doctor's instructions and finish all your tablets, even if you feel better. This is because you need to take the medicine for as long as your doctor has prescribed to get rid of all the bacteria. The US Centers for Disease Control and Prevention web site offers a comprehensive information regarding bacterial infections.