A bacterial disease is a disease caused by bacteria or their products. Many bacteria have no effect and some are beneficial, while only a small number lead to disease. This may be a result of bacterial growth, the inflammation in response to it, or of toxins (e.g., tetanus, botulism, cholera). Bacteria may be contracted from the environment, other animals or humans, or from other parts of a single individual. Infection of skin and soft tissues with staphylococcus or streptococcus leads to boils, carbuncles, impetigo, cellulitis, scarlet fever, and erysipelas.
Abscess represents the localization of bacteria, while bacteremia and (typically) sepsis are conditions in which a bacterial infection is circulating in the blood. Sometimes a specific bacterium causes a specific disease (e.g., anthrax, diphtheria, typhoid), but any bacteria in some organs cause a similar disease: in lungs, pneumonia occurs; in the urinary tract, cystitis or pyelonephritis, and in the brain coverings, meningitis. Many venereal diseases are due to bacteria. In some diseases (e.g., tuberculosis, leprosy, rheumatic fever), many manifestations are due to hypersensitivity (see immunity) to the bacteria. While antibiotics have greatly reduced death and ill-health from bacteria and vaccination against specific diseases (e.g., whooping cough) has limited the number of cases, bacteria remain an important factor in disease.