What tuberculosis is and how it starts

Tuberculosis (TB) is a bacterial infection caused by a microorganism called Mycobacterium tuberculosis. The bacteria usually attack the lungs first, but can spread to other parts of the body including the kidneys, spine, and brain. TB spreads through the air when a person with active disease coughs, sneezes, or speaks, releasing tiny droplets that another person breathes in.

Not everyone who breathes in TB bacteria becomes sick. Your immune system may kill the bacteria, or the infection may stay dormant in your body for years or even a lifetime. This is called latent TB infection. Only about 5 to 10 percent of people with latent TB will develop active disease at some point.

Active TB disease means the bacteria are multiplying and causing symptoms. This is the form that spreads to others and requires treatment. The difference between latent and active TB is crucial because only active TB makes you contagious.

Key Takeaways

  • Tuberculosis is caused by bacteria that spread through the air and usually infect the lungs first, though they can reach other organs.
  • Latent TB infection means bacteria are in your body but dormant, causing no symptoms and not spreading to others.
  • Active TB disease causes symptoms like persistent cough, chest pain, and fever, and is contagious to people nearby.
  • Risk factors for developing active TB include weakened immunity, recent TB infection, and certain medical conditions like diabetes or HIV.
  • TB is treatable with antibiotics taken over several months, but the bacteria can develop resistance if treatment is incomplete.

How TB bacteria damage the lungs

When TB bacteria settle in the lungs, they multiply and create inflammation. This inflammation damages lung tissue and forms cavities—hollow spaces where bacteria thrive. As the infection progresses, these cavities can rupture and spread bacteria to other parts of the lungs or into the bloodstream.

The damage happens gradually. Early on, you might have no symptoms at all. As the infection worsens, the inflammation and tissue damage trigger the symptoms people recognize: a cough that lasts more than three weeks, chest pain when breathing or coughing, and sometimes coughing up blood or blood-tinged sputum.

The lungs' job is to exchange oxygen for carbon dioxide in the blood. When TB destroys lung tissue, this exchange becomes less efficient. Over time, untreated TB can cause severe breathing problems and respiratory failure.

Latent versus active TB disease

The distinction between latent and active TB determines whether you need treatment and whether you can spread the disease. Latent TB infection means TB bacteria are present in your body but enclosed by scar tissue, unable to multiply or cause damage. You have no symptoms, feel well, and cannot transmit the bacteria to others. A skin test or blood test can detect latent TB, but a chest X-ray looks normal.

Active TB disease means the bacteria are actively multiplying and destroying tissue. You develop symptoms—cough, fever, night sweats, weight loss, fatigue—and you can spread the disease by breathing. A chest X-ray shows the damage, and sputum tests (samples of mucus you cough up) can detect the bacteria directly.

Some people develop active TB within weeks of infection, especially if their immune system is weak. Others carry latent TB for decades and never develop active disease. Certain conditions—HIV infection, diabetes, kidney disease, or medications that suppress immunity—increase the risk that latent TB will progress to active disease.

Why TB spreads through the air

TB spreads differently from diseases spread by touch or contaminated food. When someone with active TB coughs or sneezes, they release microscopic droplets containing TB bacteria. These droplets float in the air for hours in poorly ventilated spaces. If you breathe in these droplets, the bacteria can lodge in your lungs and begin to grow.

Close, prolonged contact increases transmission risk. Sharing a home, workplace, or classroom with someone who has active TB for weeks or months raises your chances of infection. Brief encounters with an infected person—passing someone on the street—rarely transmit TB. Healthcare workers, family members, and others in frequent contact with an untreated TB patient face higher risk.

Once someone starts taking TB antibiotics, they typically become non-contagious within two weeks, even if treatment is not yet complete. This is why early detection and treatment protect both the infected person and their contacts.

TB in different parts of the body

Although TB usually starts in the lungs, the bacteria can travel through the bloodstream and infect other organs. Extrapulmonary TB—TB outside the lungs—can affect the lymph nodes, bones, joints, kidneys, brain, and other tissues.

TB of the spine (called Pott's disease) causes back pain and can lead to spinal deformity if untreated. TB of the brain and spinal cord (tuberculous meningitis) is one of the most serious forms and can cause permanent neurological damage. TB of the lymph nodes is common and may be the only sign of infection in some people, especially those with HIV.

Extrapulmonary TB is harder to diagnose because symptoms vary by location and may not point obviously to TB. A person with TB of the kidneys might have blood in their urine. Someone with TB of the bones might have joint pain. Diagnosis often requires imaging tests, biopsies, or cultures from the affected site.

Risk factors for developing active TB

Certain conditions and circumstances make it more likely that TB infection will progress to active disease. HIV infection is the strongest risk factor—people with HIV and TB co-infection have a much higher chance of developing active TB and face more severe illness. Recent TB infection (within the past two years) also increases risk because the immune system has not yet fully contained the bacteria.

Medical conditions that weaken immunity raise risk as well: uncontrolled diabetes, chronic kidney disease, malnutrition, and conditions requiring immunosuppressive medications. Age matters too—very young children (under 5) and older adults have higher rates of progression. People who use alcohol or inject drugs also face elevated risk, partly due to weakened immunity and partly due to conditions like malnutrition and poor access to healthcare.

Living in crowded conditions—prisons, homeless shelters, nursing homes—increases both the chance of infection and the chance of progression. Healthcare workers and others with occupational exposure to TB patients also face higher risk.

How TB is diagnosed

Diagnosis depends on whether TB is latent or active. For latent TB, two main tests exist: the tuberculin skin test (also called the Mantoux test), where a small amount of TB protein is injected under the skin and checked 48 to 72 hours later, and interferon-gamma release blood tests (IGRAs), which measure immune response to TB antigens.

For active TB disease, diagnosis usually begins with a chest X-ray, which shows characteristic patterns of lung damage. The definitive test is a sputum smear microscopy or culture—examining mucus you cough up under a microscope or growing the bacteria in a lab. These tests confirm TB and can also show whether the bacteria are drug-resistant.

Because TB symptoms overlap with other lung diseases, diagnosis is not always immediate. A persistent cough prompts testing, but results take time—cultures can take weeks to grow. This is why people with TB symptoms should see a healthcare provider promptly rather than waiting to see if symptoms resolve on their own.

Treatment and why it matters

Active TB is treatable with a combination of antibiotics, typically taken for six months. The standard regimen uses four drugs in the first two months, then two drugs for the remaining four months. This combination approach works because it prevents the bacteria from developing resistance to any single drug.

Completing the full course of treatment is essential. If you stop early, some bacteria may survive and develop resistance, creating drug-resistant TB that is much harder to treat. Latent TB can also be treated with preventive antibiotics to reduce the risk of progression to active disease, especially in people with HIV or recent TB infection.

Without treatment, active TB can cause severe lung damage, spread to other organs, and be fatal. With treatment, most people recover fully. The key is starting treatment early and taking all doses as prescribed, even after symptoms disappear.

Frequently Asked Questions

Can you catch TB from someone with latent TB?

No. Latent TB is not contagious. The bacteria are dormant and enclosed in scar tissue, so they are not released into the air. Only people with active TB disease can spread the infection to others.

How long does it take to develop active TB after infection?

It varies widely. Some people develop active TB within weeks or months of infection, especially if their immune system is weak. Others may carry latent TB for years or decades and never develop active disease. On average, about 5 to 10 percent of people with latent TB will develop active disease at some point in their life.

Is TB always in the lungs?

TB usually starts in the lungs, but bacteria can spread to other parts of the body including the spine, brain, kidneys, and bones. This is called extrapulmonary TB. Symptoms depend on which organ is infected, which can make diagnosis more difficult.

What happens if TB bacteria become drug-resistant?

Drug-resistant TB occurs when bacteria survive antibiotics, usually because treatment was incomplete or interrupted. These strains require longer treatment with different, stronger drugs that have more side effects. Multi-drug-resistant TB (MDR-TB) is much harder to cure and requires 18 to 20 months of treatment instead of six.

Can TB come back after treatment?

True relapse—the same infection returning—is rare if you complete treatment fully. However, you can be infected with TB again if you are exposed to someone else with active disease. People who have had TB before should be tested if they develop symptoms again.