Tuberculosis is a bacterial infection that usually attacks the lungs
Tuberculosis (TB) is an infection caused by bacteria called Mycobacterium tuberculosis. The bacteria spread through the air when an infected person coughs, sneezes, or speaks. Most people who breathe in the bacteria don't get sick right away — their immune system stops the infection. But some people develop active TB disease, where the bacteria multiply and damage lung tissue. Without treatment, TB can spread to other parts of the body and become life-threatening.
TB is different from many other infections because it can hide in your body for years without causing symptoms. A person can have TB bacteria in their body but not be contagious and not feel sick. This is called latent TB infection. Active TB disease is when the bacteria are multiplying and causing illness — this is the form that spreads to other people.
Key Takeaways
- Tuberculosis spreads through the air when someone with active TB coughs or sneezes, but most people exposed to TB bacteria never develop the disease.
- Latent TB means you carry the bacteria but have no symptoms and cannot spread it to others, while active TB causes illness and is contagious.
- Doctors diagnose TB using a skin test, blood test, or chest X-ray, depending on your symptoms and exposure history.
- TB is treatable with antibiotics taken over several months, and treatment prevents the disease from progressing and spreading to others.
How TB spreads and who is at higher risk
TB spreads only through the air — you cannot catch it from touching someone, sharing food or drinks, or shaking hands. When a person with active TB in their lungs coughs or sneezes, they release tiny droplets containing the bacteria. If you breathe in these droplets, the bacteria can lodge in your lungs and begin to grow.
Not everyone exposed to TB gets sick. Your risk depends on your immune system and how much bacteria you were exposed to. People at higher risk of developing active TB include those with HIV or AIDS, people taking medications that weaken immunity, very young children, older adults, and people with conditions like diabetes or kidney disease. People who live or work closely with someone who has active TB — such as family members or healthcare workers — also face higher risk.
Symptoms of active tuberculosis
Active TB usually develops slowly. Early symptoms include a persistent cough lasting more than three weeks, chest pain when breathing or coughing, and coughing up blood or bloody mucus. You may also feel tired, lose weight without trying, have a fever (usually low-grade, especially in the afternoon), and sweat heavily at night.
These symptoms can be mistaken for other illnesses like pneumonia or bronchitis, which is why a doctor's evaluation is important. If TB spreads beyond the lungs to other parts of the body — such as the bones, spine, brain, or lymph nodes — symptoms will vary depending on which organs are affected. Any cough lasting more than three weeks should be checked by a doctor.
How doctors diagnose tuberculosis
Doctors use three main methods to diagnose TB. The tuberculin skin test (also called a Mantoux test) involves injecting a small amount of TB protein under the skin on your forearm. A nurse or doctor checks the injection site 48 to 72 hours later. A raised bump at the site suggests TB infection, though further testing is needed to confirm active disease.
A blood test (called an interferon-gamma release assay, or IGRA) detects TB infection by measuring how your immune cells respond to TB proteins. This test is done in a lab and results come back within a few days. If either the skin test or blood test is positive, your doctor will order a chest X-ray to look for signs of TB in your lungs. A sputum test — where you cough up mucus into a cup — may also be done to check for TB bacteria and confirm active disease.
A positive skin test or blood test alone does not mean you have active TB disease. It means you have TB infection. Your doctor uses the X-ray, sputum results, and your symptoms to determine whether you have latent or active TB.
The difference between latent and active TB
Latent TB infection means TB bacteria are in your body but dormant — not multiplying or causing damage. You have no symptoms, feel well, and cannot spread TB to others. However, latent TB can progress to active disease later, especially if your immune system weakens. People with latent TB are often treated with antibiotics to prevent this progression.
Active TB disease means the bacteria are multiplying and causing illness. You have symptoms, may feel very sick, and can spread TB to others through the air. Active TB requires prompt treatment with multiple antibiotics. Without treatment, active TB can cause permanent lung damage, spread to other organs, and be fatal.
How TB is treated
TB is treated with antibiotics, usually a combination of four drugs taken together for the first two months, followed by two drugs for the remaining four months. The most common drugs are isoniazid, rifampin, pyrazinamide, and ethambutol. Treatment takes at least six months total, and it is important to take all doses exactly as prescribed, even after you start feeling better.
If you stop taking antibiotics early, the bacteria can develop resistance to the drugs, making TB much harder to treat. Your doctor will monitor your progress with follow-up X-rays and tests. Most people with active TB stop being contagious after about two weeks of treatment, though they must continue taking all medications for the full course. Latent TB is also treated with antibiotics — usually one or two drugs taken for three to nine months — to prevent it from becoming active disease.
What happens after TB treatment
After you complete TB treatment, your doctor will do follow-up tests to confirm the infection has cleared. You may have periodic X-rays or sputum tests to make sure the bacteria are gone. Most people who complete treatment are cured and do not develop TB again, though latent TB can reactivate if your immune system becomes very weak.
If you were exposed to someone with TB but did not develop infection, your doctor may still recommend preventive antibiotics depending on your risk factors and test results. People who have been treated for TB should tell their healthcare providers about their TB history, as it can affect how certain other conditions are managed.
Frequently Asked Questions
Can you get TB from someone who has latent TB?
No. Latent TB is not contagious. Only people with active TB disease in their lungs can spread the infection to others. Someone with latent TB has no symptoms and the bacteria are not multiplying, so they cannot release bacteria into the air.
How long does it take to know if you have TB after exposure?
It can take two to eight weeks after exposure for a skin test or blood test to become positive. If you were exposed to someone with active TB, your doctor may recommend testing now and again in eight weeks if the first test is negative. Symptoms of active TB can develop weeks or months after exposure.
What if I don't finish all my TB antibiotics?
Stopping antibiotics early allows TB bacteria to survive and become resistant to the drugs. This makes TB much harder to treat and can lead to treatment failure, relapse, or the spread of drug-resistant TB to others. It is critical to take all doses for the full six months even if you feel better.
Can TB come back after treatment?
True reactivation is rare if you complete treatment correctly. However, you can be infected with TB again if you are exposed to someone else with active TB, especially if your immune system is weakened. People with HIV or other conditions that weaken immunity have higher risk of TB reactivation.
Is TB preventable?
A vaccine called BCG (Bacille Calmette-Guérin) is used in many countries to prevent TB, though it is not routinely given in the United States. The best prevention is avoiding close contact with people who have active TB and getting tested if you have been exposed. If you have latent TB, taking preventive antibiotics stops it from becoming active disease.