Tuberculosis (TB) is a bacterial infection that primarily attacks the lungs, caused by a slow-growing bacterium called Mycobacterium tuberculosis. It spreads through the air and kills an estimated 1.25 million people worldwide each year, making it one of the deadliest infectious diseases on the planet. Around 10.8 million new cases are estimated to occur globally each year, though the disease is far less common in the United States and other high-income countries.
How TB Spreads
TB is an airborne infection. When someone with active TB in their lungs, airway, or voice box coughs, speaks, or even sings, they release tiny particles called droplet nuclei into the air. These particles are small enough to float and can remain airborne for several hours depending on ventilation and sunlight exposure. Another person breathes them in, and the bacteria travel deep into the lungs.
Not everyone who inhales the bacteria gets sick. When the particles reach the small air sacs of the lungs, immune cells called macrophages typically engulf and destroy them. But if the immune system can’t fully eliminate the bacteria, the germs begin multiplying inside those very immune cells. This is where TB infection begins, and from here, the disease can take two very different paths.
Latent TB vs. Active TB Disease
This distinction matters more than almost anything else about TB. Most people who are infected with the bacteria never feel sick at all. Their immune system walls off the bacteria, keeping them contained but alive. This is called latent TB infection. You have no symptoms, you can’t spread it to anyone, and you may never know you have it unless you’re tested. A positive result on a TB skin test or blood test is often the only sign.
Active TB disease is what happens when the bacteria break free of that containment and start multiplying. This can happen weeks after initial infection or decades later, often triggered by something that weakens the immune system. People with active TB are the ones who feel sick and, in the case of lung TB, can spread the infection to others.
Roughly 5 to 10 percent of people with latent TB will eventually develop active disease at some point in their lives, though the risk is much higher for people with weakened immune systems.
Symptoms of Active TB
Active pulmonary TB tends to come on gradually. The hallmark symptom is a persistent cough, usually lasting three weeks or longer, that often produces mucus and sometimes blood. Beyond the cough, common symptoms include:
- Fever, often low-grade
- Drenching night sweats
- Unexplained weight loss
- Fatigue and general weakness
- Chest pain
- Difficulty breathing or wheezing
The combination of a cough that won’t go away, night sweats, and weight loss is the classic pattern that raises suspicion for TB. Because these symptoms overlap with many other conditions, TB can go undiagnosed for weeks or months, during which time the person may unknowingly spread the bacteria to others.
TB can also affect parts of the body outside the lungs, including the kidneys, spine, and brain. These forms of extrapulmonary TB generally don’t spread through the air but can cause serious damage to the affected organs.
Who Is Most at Risk
Anyone can get TB, but certain groups face a significantly higher risk of developing active disease once infected. People living with HIV are especially vulnerable because the virus directly weakens the immune cells that keep TB bacteria in check. TB is in fact one of the leading causes of death among people with HIV worldwide.
Other factors that raise your risk include diabetes, kidney disease, certain cancers, malnutrition, and medications that suppress the immune system (such as those used after organ transplants or for autoimmune conditions). Smoking and heavy alcohol use also increase susceptibility. Living in or traveling to countries where TB is common, or spending time in crowded settings like shelters, prisons, or nursing homes, raises the chance of exposure in the first place.
How TB Is Diagnosed
There are two main screening tests for TB infection, and neither one can tell you whether you have latent infection or active disease on its own.
The TB skin test involves injecting a small amount of protein under the skin of your forearm. You return two to three days later, and a healthcare provider checks for a raised bump at the injection site. The test correctly identifies TB infection about 77% of the time. Its main weakness: people who received the BCG vaccine (a TB vaccine given widely outside the U.S.) often test positive even without true infection, dropping the test’s reliability in that group considerably, with specificity falling to around 59%.
TB blood tests, known as IGRAs, measure how your immune cells react to TB proteins in a lab. One version has a sensitivity of about 78%, while another reaches roughly 92%. These blood tests are less affected by prior BCG vaccination, making them more reliable for people vaccinated as children. Both types of blood test have specificity above 93%, meaning false positives are uncommon.
If either screening test is positive, a chest X-ray and sputum samples (mucus coughed up from the lungs) are used to determine whether the infection is latent or has progressed to active disease.
Treatment for TB
Active TB is treated with a combination of four antibiotics taken over six to nine months. The treatment has two phases. During the first two months, called the intensive phase, you take all four medications daily (or five days a week). This is the period that kills the bulk of the bacteria and typically makes you non-contagious within a few weeks. After that, treatment continues with two of the four drugs for an additional four to seven months.
The length of treatment is one of the biggest challenges with TB. The bacteria grow slowly and can hide inside cells, so shorter courses leave surviving bacteria that can rebound. Stopping early or skipping doses is one of the main ways drug-resistant strains develop. For this reason, many TB programs use directly observed therapy, where a healthcare worker watches you take each dose.
Latent TB infection can also be treated, usually with a simpler regimen lasting three to four months, to prevent it from ever becoming active disease. This is especially important for people with HIV or other immune-compromising conditions.
Drug-Resistant TB
When TB bacteria develop resistance to the two most effective front-line drugs, the result is multidrug-resistant TB (MDR-TB). Treatment for MDR-TB requires alternative medications that are less effective, more toxic, and must be taken for much longer, sometimes 18 months or more.
An even more concerning form, extensively drug-resistant TB (XDR-TB), resists those front-line drugs plus key backup medications. XDR-TB is rare but extremely difficult to treat and carries a higher mortality rate. Drug resistance is largely a consequence of incomplete treatment, whether from patients stopping medications early, inconsistent drug supply, or providers prescribing inadequate regimens.
The BCG Vaccine
The only available TB vaccine, BCG, has been in use since the 1920s and is given to infants in most countries where TB is common. It is not routinely used in the United States. BCG works best at protecting young children from the most severe forms of TB. In newborns, it prevents about 90% of cases of TB meningitis and disseminated TB, the deadliest childhood forms.
Protection against ordinary pulmonary TB in adults is far weaker. In older children and adults, vaccine effectiveness drops below 20%. This is why TB remains so difficult to control globally: the vaccine protects the youngest children from dying but does little to stop the chain of transmission among adolescents and adults, who drive the epidemic.