How tuberculosis infection happens

Tuberculosis spreads through the air when a person with active TB disease in their lungs or throat coughs, sneezes, speaks, or sings. The bacteria travel in tiny droplets that can float in the air for several hours. If you breathe in these droplets, the bacteria can lodge in your lungs and begin to grow. This is how most people get infected.

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 a lifetime. Whether you develop active TB disease depends on your age, overall health, and how strong your immune system is.

TB does not spread through touching, shaking hands, sharing food or drinks, or sharing clothing or bedding. You cannot get it from someone's saliva or sweat. The bacteria must reach your lungs through the air.

Key Takeaways

  • TB spreads through the air when someone with active lung or throat TB coughs, sneezes, speaks, or sings, and you breathe in the bacteria.
  • You are most at risk if you spend long periods in a closed space with someone who has untreated active TB disease.
  • Infection does not always lead to active TB disease — your immune system may control the bacteria, and you may never become sick.
  • People with weakened immune systems, young children, and those with certain medical conditions are at higher risk of developing active TB after infection.
  • TB does not spread through touch, food, drinks, or shared objects — only through breathing in bacteria from the air.

Who is at higher risk of getting infected

You are at higher risk if you spend extended time in a closed space with someone who has active TB disease of the lungs or throat. This includes living in the same household, working in the same office, or riding in a car with windows closed. The longer and closer the contact, the greater your risk.

Healthcare workers, people in shelters or prisons, and those living in crowded conditions face higher exposure risk. People who have recently arrived from countries where TB is common also have a higher chance of encountering the disease.

When infection becomes active disease

After breathing in TB bacteria, one of three things can happen. First, your immune system may kill the bacteria right away, and you never develop any infection. Second, the bacteria may stay dormant in your body — this is called latent TB infection. Third, the bacteria may multiply and cause active TB disease, usually within the first two years after infection, though it can happen decades later.

Your risk of developing active TB disease is much higher if your immune system is weakened. This includes people with HIV, those taking medications that suppress immunity, people with diabetes, and those with severe kidney disease. Children under five years old who are infected also have a higher risk of developing active disease.

People with latent TB infection have no symptoms and cannot spread TB to others. Only people with active TB disease — usually in the lungs or throat — can transmit the bacteria through the air.

Settings where transmission is most likely

TB spreads most easily in poorly ventilated indoor spaces where an infected person spends hours. Homes, workplaces, schools, and healthcare facilities are common transmission sites. Crowded shelters and prisons have higher transmission rates because many people share small spaces with limited fresh air circulation.

Well-ventilated spaces, outdoor areas, and brief encounters carry much lower risk. A single cough or sneeze in passing on the street is unlikely to transmit TB, but spending eight hours in a closed office with someone who has untreated active TB carries significant risk.

The difference between infection and active disease

Latent TB infection means TB bacteria are in your body but dormant — you have no symptoms, feel fine, and cannot spread TB to anyone else. A skin test or blood test will show you have been infected, but a chest X-ray will look normal.

Active TB disease means the bacteria are multiplying and damaging your lungs or other organs. You develop symptoms like a persistent cough lasting more than three weeks, chest pain, coughing up blood, fever, night sweats, and weight loss. Only people with active TB disease can spread the infection to others. If you have symptoms, you are contagious until you have been on treatment for about two weeks.

What to do if you have been exposed

If you have spent significant time with someone who has active TB disease, contact your doctor or local health department. They can perform a skin test or blood test to see if you have been infected. Testing is usually done two to eight weeks after exposure, though sometimes a second test is needed.

If the test shows latent TB infection, your doctor may recommend treatment to prevent the bacteria from becoming active disease. If the test is negative, you may be retested after eight weeks to account for the time it takes for infection to show up on tests.

Tell your doctor about your symptoms — any cough lasting more than three weeks, fever, night sweats, or weight loss. These could mean you have active TB disease and need treatment right away.

How treatment stops transmission

If you have active TB disease, treatment with antibiotics stops transmission quickly. After about two weeks of taking the correct medications as prescribed, you are usually no longer contagious. This is why people with active TB are asked to stay home or limit contact with others until they have been on treatment for at least two weeks.

Completing the full course of TB treatment — usually six months — is essential to cure the disease and prevent it from returning. If treatment is stopped early, the bacteria can become resistant to the medications, making the disease much harder to treat.

Frequently Asked Questions

Can you get TB from someone who is being treated?

No, not after the first two weeks of treatment. Once someone has been taking TB medications correctly for about two weeks, they are no longer contagious. Before treatment or in the first two weeks, the risk is highest.

What if I was exposed but have no symptoms?

You may have latent TB infection, which means the bacteria are in your body but not causing disease. Get tested through your doctor or health department. Many people with latent infection never develop active disease, especially if treated.

Can you get TB from someone with latent infection?

No. Latent TB infection cannot spread to other people. Only active TB disease spreads through the air. Someone with latent infection has no symptoms and poses no transmission risk.

How long after exposure do symptoms appear?

Active TB disease usually develops within the first two years after infection, but it can take much longer — sometimes decades. Some people never develop symptoms at all. This is why testing after exposure is important, even if you feel fine.

Is TB spread through food or water?

TB spreads through the air, not through food or water. You cannot get TB from eating with someone or drinking from the same cup. The bacteria must reach your lungs by breathing them in.