Tuberculosis (TB) is a bacterial infection that primarily attacks the lungs, though it can affect nearly any organ in the body. It spreads through the air when someone with active lung TB coughs, speaks, or sings. Globally, an estimated 10.8 million new cases and 1.25 million deaths occur each year, making TB one of the deadliest infectious diseases on the planet.
How TB Infects the Body
TB is caused by a slow-growing bacterium called Mycobacterium tuberculosis. What makes this organism so difficult to fight is its unusually thick, waxy cell wall, which acts as a barrier against both the immune system and many drugs. The bacterium divides only once every 12 to 24 hours, far slower than most bacteria. That slow pace is part of why TB treatment takes months rather than days.
Infection begins when tiny particles containing the bacteria, called droplet nuclei, are inhaled into the lungs. These particles can remain suspended in the air for several hours depending on ventilation. Once inside the lungs, the bacteria are swallowed up by immune cells called macrophages, which normally destroy invaders. But TB has evolved specialized tools to escape that defense. It can break out of the compartment where macrophages trap it and survive inside the cell, essentially using immune cells as a hiding place.
In most healthy people, the immune system walls off the bacteria into tiny clusters, preventing them from multiplying or causing symptoms. This is latent TB infection. The bacteria are alive but dormant. You can’t spread TB to others in this state, and you won’t feel sick. But if the immune system weakens later in life, those dormant bacteria can reactivate and cause disease.
Latent Infection vs. Active Disease
This distinction is one of the most important things to understand about TB. Someone with latent TB infection carries a small number of inactive bacteria. They have no symptoms, can’t transmit TB to anyone, and often don’t know they’re infected unless they get a skin or blood test. Roughly a quarter of the world’s population has latent TB.
Active TB disease is a different situation entirely. The bacteria are multiplying, causing damage to tissue, and producing symptoms. People with active pulmonary TB can spread the bacteria to others through the air. Without treatment, active TB is often fatal. The risk of latent TB progressing to active disease is about 5 to 10 percent over a lifetime, but that risk jumps significantly for people with weakened immune systems.
Symptoms of Active TB
TB typically comes on gradually. Symptoms start mild and build over weeks or months, which is one reason people often delay getting checked. Early on, it’s easy to chalk up a lingering cough to allergies or blame fatigue on stress. By the time someone seeks medical care, the disease may have been progressing for a while.
General symptoms that can appear regardless of where TB has settled in the body include fever, night sweats, unexplained weight loss, loss of appetite, and a persistent sense of fatigue or illness.
When TB affects the lungs, the most common form, additional symptoms include:
- A cough lasting three weeks or longer
- Coughing up mucus or blood
- Chest pain
- Shortness of breath
TB can also develop outside the lungs, known as extrapulmonary TB. The symptoms depend on the organ involved. Blood in the urine may point to kidney TB. Headache or confusion can signal TB meningitis. Back pain may indicate spinal TB. Swollen lymph nodes, hoarseness, or a painful swollen joint are other possible signs. These forms are less common but can be serious, especially TB meningitis.
How TB Is Diagnosed
Two main screening tools detect TB infection. The tuberculin skin test (TST) involves injecting a small amount of protein under the skin of the forearm and checking for a raised bump 48 to 72 hours later. It’s the most widely used TB test globally, though it can give misleading results in people who’ve received the BCG vaccine (a TB vaccine given in many countries) or who have weakened immune systems.
Blood tests called interferon-gamma release assays (IGRAs) measure the immune system’s response to TB-specific proteins. They’re more precise than the skin test in BCG-vaccinated individuals, though their sensitivity drops to about 60 to 70 percent in people with HIV. Neither the skin test nor blood tests can distinguish between latent infection and active disease on their own.
When active TB is suspected, doctors rely on chest X-rays to look for characteristic lung damage and on sputum samples (mucus coughed up from the lungs) to confirm the presence of bacteria. Molecular tests can identify TB bacteria and certain drug-resistance patterns within hours, a major improvement over older culture methods that take weeks to return results.
Treatment and Duration
Standard TB treatment uses a combination of four antibiotics taken during an intensive two-month phase, followed by a continuation phase of four to seven more months, for a total of six to nine months. Using multiple drugs simultaneously is essential because the bacteria’s slow growth and protective cell wall make it hard for any single drug to finish the job. Stopping treatment early or missing doses is one of the main ways drug-resistant strains develop.
For latent TB, shorter courses of one or two medications over three to four months can prevent the infection from ever becoming active. This is especially important for people at higher risk of progression, such as those with HIV, recent contacts of someone with active TB, or people on immune-suppressing medications.
Treatment for drug-susceptible TB is highly effective when completed. The challenge is adherence. Nine months of daily medication with potential side effects is a commitment, and many public health programs use directly observed therapy, where a health worker watches the patient take each dose, to ensure the full course is finished.
Drug-Resistant TB
When TB bacteria become resistant to the two most powerful first-line drugs, the result is multidrug-resistant TB (MDR-TB). Treatment for MDR-TB is longer, more expensive, and involves drugs with harsher side effects. An even more dangerous form, extensively drug-resistant TB (XDR-TB), resists additional classes of backup drugs as well.
Resistance develops in two ways. Primary resistance happens when someone is directly infected with an already-resistant strain. Secondary, or acquired, resistance develops during treatment when a patient doesn’t complete the full course, takes medications inconsistently, or has drug absorption problems that reduce the drugs’ effectiveness. This is why finishing the entire treatment course matters so much, not just for the individual but for the broader community.
Who Is Most at Risk
TB thrives where immune systems are compromised and where people live in close quarters with poor ventilation. HIV is the single biggest risk factor for developing active TB. People with diabetes, malnutrition, or those taking immune-suppressing drugs are also at elevated risk. Crowded living conditions, such as prisons, shelters, and densely populated housing, increase the chance of transmission.
Healthcare workers, people who have recently immigrated from countries with high TB rates, and anyone in close contact with a person who has active TB face higher exposure risk. Children under five and elderly adults are more vulnerable to severe disease once infected.
Prevention
The BCG vaccine, used widely outside the United States, offers some protection for young children against severe forms of TB like TB meningitis. Its effectiveness against pulmonary TB is modest, around 19 percent, and that benefit appears limited to young children. Researchers have not found conclusive evidence that it protects older children or adults against lung TB, which is why it’s not part of routine vaccination in many low-burden countries.
Beyond vaccination, preventing TB transmission relies on identifying and treating active cases quickly, ensuring good ventilation in shared spaces, and treating latent infections in high-risk individuals before they progress. In healthcare settings, measures like respiratory isolation rooms and fitted respirator masks help protect workers from airborne bacteria that can linger in the air for hours after an infectious person has left the room.