Tuberculosis (TB) is a serious bacterial infection that has affected humanity for millennia. It primarily attacks the lungs, but the bacteria can travel through the bloodstream to affect other parts of the body, including the spine, kidneys, and brain. Despite being a treatable and preventable disease, TB remains a major global public health challenge. The infection’s ability to lie dormant for years and the complexity of its treatment regimen contribute to its ongoing prevalence worldwide.
The Bacterium and How Tuberculosis Spreads
The illness is caused by the slow-growing bacterium Mycobacterium tuberculosis, a rod-shaped microorganism requiring high levels of oxygen to survive. This bacterium has a unique, waxy outer coating, rich in mycolic acid, which shields it from the immune system and many common antibiotics. This protective layer allows the organism to persist within the human host.
TB spreads exclusively through the air via airborne transmission, not through surface contact like sharing utensils. When a person with active TB in their lungs coughs, sneezes, or speaks, they release microscopic droplet nuclei into the air. These tiny droplets can remain suspended for several hours.
Transmission occurs when a person breathes in these airborne droplets, allowing the bacteria to settle in the lungs. TB bacteria are more likely to spread in enclosed spaces with poor ventilation, where the concentration of infectious droplet nuclei remains high. Close, prolonged contact with an infectious person significantly increases the risk of becoming infected.
Distinguishing Latent and Active Infection
After the bacteria are inhaled, the infection progresses into either latent infection or active disease. Latent TB Infection (LTBI) occurs when the immune system successfully contains the bacteria, walling them off in the lungs. In this state, the bacteria are alive but inactive, meaning they are not multiplying.
A person with latent TB is not sick, has no symptoms, and cannot spread the infection. The bacteria remain dormant and can reactivate later, especially if the immune system weakens. Without preventive treatment, approximately 5% to 10% of people with LTBI will develop active TB disease.
Active TB Disease occurs when the bacteria overcome immune defenses and multiply rapidly, causing illness and making the person contagious. Symptoms of active pulmonary TB include a persistent cough lasting three weeks or longer, chest pain, and coughing up blood or sputum. Systemic symptoms include unexplained weight loss, fever, night sweats, and fatigue. Only those with active disease in the lungs or throat can transmit the bacteria.
Modern Diagnosis and Standard Treatment Protocols
Determining a person’s infection status involves diagnostic tools that check for an immune response to the bacteria. The Tuberculin Skin Test (TST), or Mantoux PPD, involves injecting a small amount of purified protein derivative under the skin. Blood tests, known as Interferon-Gamma Release Assays (IGRAs), measure the immune cells’ reaction to TB proteins. A positive result indicates infection but does not distinguish between latent infection and active disease.
If infection is indicated, further testing is required to confirm active disease. A chest X-ray is used to look for characteristic changes in the lungs, such as infiltrates or cavities. Definitive diagnosis involves collecting a sputum sample, which is then examined through microscopy, culture, or rapid molecular tests to identify Mycobacterium tuberculosis. These molecular tests can also quickly detect genetic mutations that signal resistance to certain antibiotics.
Treating drug-susceptible active TB is a lengthy process requiring a multi-drug regimen of four different antibiotics taken simultaneously over four to nine months. The initial phase uses four drugs—isoniazid, rifampin, pyrazinamide, and ethambutol—to quickly kill the majority of bacteria. This is followed by a continuation phase using fewer drugs to eliminate remaining, slower-growing organisms. Strict adherence to this long course is paramount, as stopping treatment prematurely can lead to drug-resistant TB (MDR-TB), which is much harder to treat and may require 18 to 24 months of therapy.
Global Impact and Prevention Strategies
TB is a major global cause of death, concentrated in low- and middle-income countries, and disproportionately affects vulnerable populations. Control relies on effective case finding, treatment adherence, and vaccination. The Bacillus Calmette-Guérin (BCG) vaccine is the only available TB vaccine and protects infants against severe forms like meningitis, but its effectiveness against pulmonary TB in adults is limited.
Public health efforts focus on rapidly diagnosing and isolating individuals with active TB to stop airborne spread. Improving ventilation in high-risk settings, such as hospitals, reduces infectious particle concentration. Treating latent TB infection is also a strategy to prevent progression to active disease.