A virus is one of the two major infectious agents. Viruses cannot be controlled with antibiotics. If your natural defenses are weakened by stress, lack of rest and poor diet, your body may not be able to resist a viral attack. Viral infections such as influenza, colds, and mononucleosis do not improve with treatment by penicillin or other antibiotics.
Bacteria are the second type of the major infectious agents.
Most bacterial infections are easily treated. Common bacterial infections such as bronchitis and gastroenteritis often occur during or after a viral infection due to the body's weakened condition. These and many other types of infections such as bladder infections, sexually transmitted diseases, pink eye, boils, and ear infections often require treatment with an antibiotic. Antibiotics are medicines used in the treatment of infections caused by bacteria. There are many different types of antibiotics and each can be used to treat a different kind of infection.