Bacterial diseases of humans can be discussed according to their mode of acquisition/transmission. Most of the airborne diseases caused by bacteria involve the respiratory system. Examples include diphtheria, Legionnaires’ disease and Pontiac fever, Mycobacterium avium–M, intracellulare pneumonia, pertussis, streptococcal diseases, and tuberculosis. Other airborne bacteria can cause skin diseases, including cellulitis, erysipelas, and scarlet fever, or systemic diseases such as meningitis, glomerulonephritis, and rheumatic fever.
Although arthropod-borne bacterial diseases are generally rare, they are of interest either historically (plague) or because they have been newly introduced into humans (Lyme disease). Most of the rickettsial diseases are arthropod-borne. The rickettsias found in the United States can be divided into the typhus group (epidemic typhus caused by R. prowazekii and murine typhus caused by R. typhi) and the spotted fever group (Rocky Mountain spotted fever caused by R. rickettsii and ehrlichiosis caused by Ehrlichia chaffeensis), with Q fever (caused by Coxiella burnetti) being an exception because it forms endosporelike structures and does not have to use an insect vector as with other rickettsias.
Most of the direct contact bacterial diseases involve the skin, mucous membranes, or underlying tissues. Examples include anthrax, bacterial vaginosis, cat-scratch disease, chancroid, gas gangrene, leprosy, peptic ulcer disease and gastritis, staphylococcal diseases, and syphilis. Others can become disseminated throughout specific regions of the body—for example, gonorrhea, staphylococcal diseases, syphilis, tetanus, and tularemia. Three chlamydial species cause direct contact disease: Chlamydia pneumonia causes chlamydial pneumonia; Chlamydia trachomatis causes inclusion conjunctivitis, lymphogranuloma venereum, nongonococcal urethritis, and trachoma; and C. psittaci causes psittacosis. Three species of mycoplasmas are human pathogens: Mycoplasma hominis and Ureaplasma urealyticum cause genitourinary tract disease, whereas M. pneumoniae is a major cause of acute respiratory disease and pneumonia.
The food-borne and waterborne bacterial diseases are contracted when contaminated food or water is ingested. These diseases are essentially of two types: infections and intoxications. An infection occurs when a pathogen enters the gastrointestinal tract and multiplies. Examples include Campylobacter gastroenteritis, salmonellosis, listerosis, shigellosis, traveler’s diarrhea, Escherichia coli infections, and typhoid fever. An intoxication occurs because of the ingestion of a toxin produced outside the body. Examples include botulism, cholera, and staphylococcal food poisoning.
Some microbial diseases and their effects cannot be categorized under a specific mode of transmission. Two important examples are sepsis and septic shock. Gram-positive bacteria, fungi, and endotoxin-containing gram-negative bacteria can initiate the pathogenic cascade of sepsis leading to septic shock. Several bacterial odontopathogens are responsible for the most common bacterial diseases in humans—tooth decay and periodontal disease. Both are the result of plaque formation and the production of lactic and acetic acids by the odontopathogens.