Tuberculosis (TB) is a bacterial infection caused by Mycobacterium tuberculosis that primarily attacks the lungs, though it can spread to virtually any organ in the body. It remains one of the world’s deadliest infectious diseases, killing an estimated 1.25 million people and infecting 10.8 million more each year. Despite being preventable and curable, TB thrives in conditions of poverty, overcrowding, and weakened immune systems.
How TB Enters and Infects the Body
TB spreads through the air. When someone with active lung TB coughs, sneezes, or even talks, they release tiny droplets called aerosols that contain the bacteria. If you inhale those particles, the bacteria travel deep into your lungs, reaching the small air sacs (alveoli) where oxygen exchange happens.
Once there, the bacteria encounter immune cells called macrophages, whose job is to swallow and destroy invaders. TB has evolved a workaround: it gets engulfed by these macrophages but then multiplies inside them, essentially using the body’s own defense system as a hiding place. Over time, the immune system walls off the infected area into clumps of tissue called granulomas. These structures contain the bacteria but don’t always kill them, which is why TB can linger in the body for years or even decades without causing symptoms.
Latent TB vs. Active TB
Not everyone infected with TB gets sick. The infection exists in two distinct forms, and understanding the difference matters for your health and the health of people around you.
Latent TB means the bacteria are in your body but your immune system is keeping them in check. You feel completely fine, have no symptoms, and cannot spread TB to anyone else. Most people who get infected stay in this stage. The lifetime risk of latent TB reactivating into active disease is relatively low: about 5% of otherwise healthy adults develop active TB within the first two years of infection. After that initial window, the annual risk drops dramatically, to roughly 10 to 20 cases per 100,000 people with long-established latent infection.
Active TB is the disease itself. The bacteria are multiplying, causing tissue damage, and you’re likely contagious if the infection is in your lungs. Active TB requires treatment. It can develop shortly after initial infection or years later if the immune system weakens due to aging, HIV, malnutrition, or other conditions.
Symptoms of Active TB
Because TB most commonly affects the lungs, the hallmark symptom is a persistent cough lasting three weeks or longer. That cough may produce phlegm or blood. Other lung-related symptoms include chest pain and shortness of breath.
TB also causes systemic symptoms that affect the whole body:
- Unexplained weight loss and loss of appetite
- Fever and chills
- Drenching night sweats
- Fatigue and weakness
These symptoms tend to develop gradually, which is one reason TB often goes undiagnosed for weeks or months. The slow onset can make it easy to dismiss as a lingering cold or general run-down feeling.
TB Beyond the Lungs
While the lungs are the primary target, TB bacteria can travel through the bloodstream and infect other parts of the body. This is called extrapulmonary TB. It can affect the lymph nodes, bones, joints, kidneys, brain, and the lining around the heart or spine. The general symptoms of fever, night sweats, weight loss, and fatigue apply regardless of where in the body the infection settles, but the specific symptoms vary by location. TB in the spine causes back pain, TB in the kidneys may show up as blood in the urine, and TB meningitis (infection of the brain’s lining) causes headaches and confusion. Extrapulmonary TB is more common in people with weakened immune systems.
How TB Is Diagnosed
Two main screening tests detect TB infection. The tuberculin skin test (TST) involves injecting a small amount of protein under the skin of your forearm. You return two to three days later so a healthcare provider can check for a raised bump at the injection site. An induration greater than 5 mm is generally considered positive, though the threshold varies based on your risk factors.
The alternative is a blood test called an interferon-gamma release assay (IGRA). Both tests are accurate for detecting latent TB, with the skin test showing about 77% overall sensitivity and the blood test around 78 to 92%, depending on the specific version used. The blood test has one clear advantage: it isn’t affected by prior BCG vaccination, which is the TB vaccine given widely outside the United States. The skin test can produce false positives in people who received that vaccine, particularly if they were vaccinated after infancy.
Neither test, however, can reliably predict whether a latent infection will progress to active disease. If active TB is suspected, chest X-rays and sputum samples (coughed-up mucus examined under a microscope and cultured for bacteria) are used to confirm the diagnosis.
Treatment
Standard TB treatment uses a combination of four antibiotics taken in two phases. The intensive phase lasts about two months and uses all four drugs together. After that, a continuation phase of four to seven additional months uses fewer drugs. Total treatment typically runs six to nine months.
The length and complexity of this regimen is one of TB’s biggest challenges. Feeling better after a few weeks tempts many patients to stop early, but doing so risks a relapse and, more critically, can breed drug-resistant bacteria. Completing the full course is essential.
Latent TB is also treatable, typically with fewer drugs over a shorter period. Treating latent TB significantly reduces the chance it will ever become active, which is especially important for people at higher risk of progression, like those with HIV or recently exposed to an active case.
Drug-Resistant TB
When TB bacteria develop resistance to the two most powerful frontline antibiotics, the result is multidrug-resistant TB (MDR-TB). This is a serious global concern. In 2023, approximately 400,000 new TB cases were drug-resistant, leading to an estimated 150,000 deaths.
MDR-TB requires longer, more complex treatment with second-line drugs that are often less effective and cause more side effects. The global treatment success rate for MDR-TB was 63% in 2020, a notable improvement from 50% in 2012, but still far below the success rates for standard TB. An even more resistant form, called extensively drug-resistant TB (XDR-TB), is resistant to additional backup drugs, leaving very few treatment options.
Drug resistance almost always develops when treatment is incomplete, inconsistent, or uses the wrong drugs. This is why directly observed therapy, where a healthcare worker watches the patient take each dose, is a cornerstone of TB control programs worldwide.
Prevention and the BCG Vaccine
TB spreads most efficiently in enclosed, poorly ventilated spaces. Good airflow is one of the simplest and most effective ways to reduce transmission risk. In healthcare settings, infection control relies on isolating infectious patients in specially ventilated rooms, using air filtration systems, and requiring fitted respirator masks for staff entering those rooms.
The BCG vaccine, named after the weakened strain of bacteria it contains, is the only available TB vaccine. It’s given to infants in many countries where TB is common, primarily to prevent the most dangerous forms of childhood TB: disseminated (widespread) TB and TB meningitis. Its effectiveness against these severe forms is estimated at about 78% in newborns. Against TB disease overall, efficacy is more modest, around 51% across studies, and clinical trials have produced conflicting results about how well it protects adults against pulmonary TB. For this reason, the BCG vaccine is not routinely used in countries with low TB rates, including the United States.
The most practical prevention strategy for individuals remains identifying and treating latent TB before it becomes active and contagious, breaking the chain of transmission that has kept this ancient disease circulating for thousands of years.