What causes tuberculosis?
Tuberculosis is caused by infection with germs called Mycobacterium tuberculosis. These bacteria were first identified in 1882 by Robert Koch who described their appearance under the microscope as rod-shaped germs or bacilli. Tuberculosis can also afflict cattle, where the infection is caused by a similar type of bacterium called Mycobacterium bovis. It is believed that tuberculosis in humans has been around since cattle were domesticated around 10,000 years ago. TB germs from
cattle can be transmitted to humans through drinking heavily infected milk. This is now rare as Western farmers have virtually eliminated the disease from their herds and pasteurization eliminates any residual bacilli.
Tuberculosis spreads by droplet infection. This type of transmission means that when a TB patient exhales, coughs, or sneezes, tiny droplets of fluid containing tubercle bacilli are released into the air. This mist, or aerosol as it is often called, can be taken into the nasal passages and lungs of a susceptible person nearby. Tuberculosis is not, however, highly contagious compared to some other infectious diseases. Only about one in three close contacts of a TB patient, and fewer than 15% of more remote contacts are likely to become infected. As a rule, close, frequent, or prolonged contact is needed to spread the disease. Of course, if a severely infected patient emits huge numbers of bacilli, the chance of transmitting infection is much greater. Unlike many other infections, TB is not passed on by contact with a patient's clothing, bed linens, or dishes and cooking utensils. The most important exception is pregnancy. The fetus of an infected mother may contract TB by inhaling or swallowing the bacilli in the amniotic fluid.
Once inhaled, tubercle bacilli may reach the small breathing sacs in the lungs (the alveoli), where they are taken up by cells called macrophages. The bacilli multiply within these cells and then spread through the lymph vessels to nearby lymph nodes. Sometimes the bacilli move through blood vessels to distant organs. At this point they may either remain alive but inactive (quiescent), or they may cause active disease. Actual tissue damage is not caused directly by the tubercle bacillus, but by the reaction of the person's tissues to its presence. In a matter of weeks the host develops an immune response to the bacillus. Cells attack the bacilli, permit the initial damage to heal, and prevent future disease permanently.
More information on tuberculosis
What is tuberculosis? - Tuberculosis (TB) is a contagious, wasting disease characterized by the coughing up of mucus and sputum, fever, weight loss, and chest pain.
How is tuberculosis transmitted? - Tuberculosis is spread through air droplets which are expelled when persons with infectious TB disease cough, sneeze, speak, or sing.
What causes tuberculosis? - Tuberculosis is caused by the bacterium Mycobacterium tuberculosis. Transmission occurs only from persons with active TB disease.
What're the risk factors for tuberculosis? - Tuberculosis is more common in elderly persons. The risk of tuberculosis increases when the immune system is unhealthy.
What are the symptoms of tuberculosis? - Symptoms of tuberculosis include cough, sputum, bleeding from the lungs, fever, night sweats, loss of weight, and weakness.
How is tuberculosis diagnosed? - Diagnosis of tuberculosis includes a medical history, a physical examination, a tuberculin skin test, a chest X-ray, and microbiologic smears and cultures.
What's the treatment for tuberculosis? - Directly observed treatment is effective in eliminating the problem of noncompliance. Surgical treatment of tuberculosis may be used if medications are ineffective.
How to prevent tuberculosis? - Preventive measures for tuberculosis include strict standards for ventilation, air filtration, and isolation methods. Preventive antibiotic treatment may have to be given.