Tuberculosis spreads through the air when an infected person coughs, sneezes, or speaks

Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis. You get it by breathing in these bacteria when someone with active TB disease releases them into the air. This happens when they cough, sneeze, speak, or sing. The bacteria travel in tiny droplets that float in the air for several hours.

You cannot get TB from shaking hands, sharing food or drinks, touching someone's skin, or sharing clothing or bedding. You also cannot get it from touching a doorknob or toilet seat that an infected person has used. TB requires breathing in the bacteria directly — it is a respiratory infection, not a contact infection.

Not everyone who breathes in TB bacteria becomes sick. Whether you develop the disease depends on how much bacteria you inhale, how long you are exposed, and how strong your immune system is.

Key Takeaways

  • TB spreads through the air when someone with active TB disease coughs, sneezes, or speaks, releasing bacteria-filled droplets that others breathe in.
  • You are most likely to catch TB if you spend time in close quarters with someone who has active disease — such as living in the same home or working in the same small room for hours.
  • People with TB infection in their lungs or throat are contagious; those with TB in other parts of the body usually are not.
  • Your immune system may stop the bacteria before you become sick, which is called TB infection; only some people with infection go on to develop active TB disease.

Who is most likely to catch TB

You have a higher risk of catching TB if you spend prolonged time near someone with active disease. This includes people who live in the same household, work in the same office or classroom for many hours, or share a small enclosed space like a car or bus regularly. Healthcare workers and people in correctional facilities also face higher exposure.

Certain groups are more likely to develop active TB disease after breathing in the bacteria. These include people with weakened immune systems (from HIV, certain medications, or organ transplants), people with diabetes, and people who smoke or misuse alcohol. Children under 5 years old and adults over 65 are also at higher risk.

If you have recently arrived in the United States from a country where TB is common, your risk is higher. TB rates are much higher in parts of Africa, Asia, Eastern Europe, and Latin America than in the United States.

The difference between TB infection and TB disease

When you breathe in TB bacteria, one of two things can happen. Your immune system may fight off the bacteria and stop them from growing. In this case, you have TB infection (also called latent TB) — the bacteria are in your body but dormant, and you have no symptoms. You cannot spread TB to others if you have only infection.

If your immune system cannot stop the bacteria, they multiply and cause illness. This is called active TB disease. You develop symptoms like a persistent cough, fever, and night sweats. People with active TB disease in the lungs or throat are contagious and can spread the disease to others.

About 5 to 10 percent of people with TB infection will develop active disease at some point in their lives. The risk is highest in the first two years after infection. People with weakened immune systems are much more likely to progress from infection to disease.

How long you need to be exposed to catch TB

There is no set time limit — it depends on how much bacteria the sick person is releasing and how close you are to them. Generally, you are more likely to catch TB if you spend several hours per day in the same room as someone with active disease. A single brief encounter, like passing someone on the street, is unlikely to transmit TB.

People with active TB disease are most contagious in the first two to three weeks after symptoms start, before they begin treatment. Once someone with TB starts taking the right medications, they usually become non-contagious within two weeks, even if they still have symptoms.

Healthcare settings can pose a higher risk because people with undiagnosed TB may be in close contact with staff and other patients. This is why hospitals and clinics have procedures to identify and isolate people with suspected TB quickly.

Settings where TB transmission is more common

TB spreads more easily in crowded, poorly ventilated spaces where people spend many hours together. Homeless shelters, correctional facilities, and nursing homes have seen TB outbreaks for this reason. Shared housing with poor air circulation — such as overcrowded apartments or dormitories — also increases transmission risk.

Healthcare facilities, especially emergency departments and intensive care units, can be sites of transmission if TB is not suspected and the person is not isolated. Schools and workplaces pose lower risk unless someone with active disease spends many hours in a small, enclosed room with poor ventilation.

Public transportation, stores, and restaurants carry minimal risk because exposure is usually brief and the space is larger. The key factor is time spent in close proximity in a confined space.

What happens after you are exposed

If you have been exposed to someone with active TB disease, you will not know immediately whether you have caught the infection. It can take two to eight weeks for your immune system to react enough for a TB test to detect infection. During this window, you may have no symptoms and no way to know you have been exposed.

Your doctor may recommend a TB test if you know you have been in close contact with someone who has active disease. A skin test or blood test can show whether you have TB infection. If the test is positive, your doctor will likely order a chest X-ray to check whether you have active disease or only infection.

If you have TB infection but not disease, your doctor may recommend preventive treatment — usually a single antibiotic taken for several months. This can reduce your risk of developing active disease by about 90 percent. Preventive treatment is especially important if you have a weakened immune system or other risk factors.

Symptoms that mean someone is contagious

A person with active TB disease in the lungs or throat usually has a cough that lasts more than three weeks. They may also have chest pain, cough up blood or blood-tinged sputum, fever, chills, and night sweats. These symptoms mean the bacteria are multiplying in the lungs and the person is likely contagious.

If someone has TB in other parts of the body — such as the bones, lymph nodes, or kidneys — they usually are not contagious because the bacteria are not in the lungs or throat. However, TB in the lungs is the most common form and the form that spreads to others.

If you think you have been exposed to someone with TB symptoms, tell your doctor. Do not wait for a cough to last three weeks — mention any exposure you know about, even if the other person has not been diagnosed yet.

Frequently Asked Questions

Can you catch TB from someone who is taking TB medicine?

Once someone starts taking the correct TB medications, they usually stop being contagious within about two weeks. However, they may still have a cough and other symptoms for longer. If you know someone is being treated for TB, the risk to you is very low after they have been on medicine for two weeks.

What should I do if I think I was exposed to TB?

Contact your doctor and tell them about the exposure. Your doctor can order a TB test to check whether you have infection. You may need a skin test or blood test now and another test eight to ten weeks later, since it takes time for infection to show up on tests.

Can you get TB from eating food prepared by someone with TB?

No. TB bacteria do not survive in the stomach, and you cannot catch TB by eating food or drinking beverages prepared by someone with TB. The bacteria must reach your lungs to cause infection.

Is TB spread through saliva or spit?

TB is not spread through saliva alone. It spreads through respiratory droplets released when someone coughs or sneezes. These droplets are different from saliva and travel through the air rather than through direct contact with spit.

Can you catch TB from someone who has only TB infection, not disease?

No. People with TB infection have dormant bacteria in their bodies, but the bacteria are not multiplying or being released into the air. Only people with active TB disease in the lungs or throat are contagious.