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

You get tuberculosis (TB) by breathing in bacteria called Mycobacterium tuberculosis that another person releases into the air. When someone with active TB in their lungs or throat coughs, sneezes, speaks, or sings, they send tiny droplets containing the bacteria into the air around them. If you breathe in those droplets, the bacteria can lodge in your lungs and begin to grow.

Not every exposure leads to infection. The bacteria need to reach your lungs — they cannot infect you through your skin or by touching contaminated surfaces. You also cannot get TB from sharing food, drinks, utensils, or a bed with someone who has the disease. The infection happens only through airborne droplets, and usually only after repeated or prolonged contact with a person who has active TB disease.

The risk is highest in enclosed spaces with poor ventilation, where infected droplets linger longer in the air. Crowded indoor settings — prisons, homeless shelters, hospitals, nursing homes, or homes with poor airflow — create conditions where TB spreads more easily than it would in open air or well-ventilated spaces.

Key Takeaways

  • TB spreads only through the air when a person with active TB disease coughs, sneezes, or speaks, not through touch or shared objects.
  • You must breathe in the bacteria for infection to occur; the droplets cannot infect you through skin or other routes.
  • Repeated or prolonged exposure to someone with active TB disease increases your risk of infection far more than a single brief encounter.
  • Enclosed spaces with poor ventilation create higher risk because infected droplets stay in the air longer and concentrate in a smaller area.
  • Having TB infection does not automatically mean you will develop TB disease; your immune system may control the bacteria for years or your whole life.

The difference between TB infection and TB disease

Breathing in the bacteria does not always mean you will get sick. Your immune system may stop the bacteria from growing, leaving you with latent TB infection — you carry the bacteria but have no symptoms and cannot spread the disease to others. This can last for years or decades without ever becoming active disease.

TB disease develops when your immune system cannot control the bacteria and they begin to multiply in your lungs. This is when you develop symptoms like a persistent cough, chest pain, fever, and night sweats, and when you can spread TB to other people. About 5 to 10 percent of people with latent infection will develop TB disease at some point in their lives, usually within the first two years after infection.

Who is at higher risk of developing TB disease after exposure

Your risk of developing TB disease after breathing in the bacteria depends on the strength of your immune system. People with weakened immune systems — including those with HIV, those taking immunosuppressant medications, people with diabetes, and very young children — are far more likely to develop active disease after exposure than people with healthy immune systems.

Other factors that increase risk include malnutrition, substance use disorder, and living in crowded conditions with poor sanitation. People who were infected with TB more than two years ago also face higher risk if their immune system weakens later in life. Age matters too: very young children (under 5) and older adults have higher rates of progression from latent infection to active disease.

How long it takes to develop symptoms after exposure

If you are exposed to TB bacteria, symptoms can appear anywhere from weeks to months after infection, though some people take much longer to show signs of disease. The average time from infection to the start of symptoms is about three to eight weeks, but this varies widely depending on your immune system and the amount of bacteria you were exposed to.

Because symptoms develop slowly, you might not realize you were exposed or infected for some time. This is why people who have been in close contact with someone diagnosed with TB disease are often tested even if they have no symptoms — catching latent infection early allows for treatment that prevents progression to active disease.

Settings where TB transmission happens most often

TB spreads most readily in places where people spend hours together in enclosed spaces. Household transmission is common: family members of someone with active TB disease face significant risk, especially if they share a bedroom or spend many hours in the same room. Healthcare settings, particularly areas where TB patients are treated, also see transmission if proper ventilation and infection control measures are not in place.

Congregate settings — prisons, jails, homeless shelters, and long-term care facilities — have documented TB outbreaks because people live in close quarters with limited privacy and ventilation. Workplaces with poor air circulation, crowded public transportation, and schools can also be sites of transmission, though the risk is lower than in homes or congregate settings where exposure is prolonged.

What happens after you breathe in TB bacteria

Once the bacteria reach your lungs, your immune system begins to respond. White blood cells surround and try to kill the bacteria, but Mycobacterium tuberculosis is difficult to destroy and can survive inside immune cells. Your body may wall off the bacteria in small nodules (called granulomas), containing them so they cannot spread or cause symptoms. This is latent TB infection.

If your immune system cannot contain the bacteria, they continue to multiply and damage lung tissue. This causes the symptoms of active TB disease — a cough that lasts more than three weeks, chest pain, coughing up blood or blood-tinged sputum, fever, chills, and night sweats. At this stage, the bacteria are active in your lungs and you can spread them to others through the air.

Testing after known exposure to TB

If you know you have been in close contact with someone who has active TB disease, you should be tested even if you have no symptoms. Two main tests can detect TB infection: a tuberculin skin test (also called a Mantoux test), where a small amount of TB antigen is injected under your skin and checked 48 to 72 hours later, and interferon-gamma release assays (IGRAs), which are blood tests that measure your immune response to TB antigens.

A positive test means you have TB infection — either latent or active — but does not tell you which one. A chest X-ray and sometimes sputum tests are used to determine whether you have active TB disease or latent infection. Testing is especially important if you have risk factors for progression, because treatment of latent TB infection can prevent you from ever developing active disease.

Frequently Asked Questions

Can you get TB from someone who does not have symptoms?

No. Only people with active TB disease in their lungs or throat can spread the bacteria. People with latent TB infection have no symptoms and cannot transmit the disease to others, even with close contact. You cannot get TB from someone unless they have active disease and are coughing or sneezing.

How long do you have to be around someone with TB to get infected?

There is no fixed time limit. Risk increases with the length and closeness of contact, but infection can occur after a single exposure in some cases, particularly in poorly ventilated spaces. Repeated or prolonged exposure — such as living in the same home or working in the same room for hours — carries much higher risk than a brief encounter.

If you are vaccinated against TB, can you still get it?

The BCG vaccine, used in many countries outside the United States, provides some protection against TB but is not completely effective. Vaccinated people can still become infected and develop TB disease, though the vaccine may reduce the severity of illness. Vaccination status does not change how TB spreads or your need for testing after exposure.

What should you do if you think you were exposed to TB?

Contact your doctor or local health department and tell them about the exposure. They can arrange testing and determine whether you have TB infection. Do not wait for symptoms to appear — testing after known exposure is the way to catch latent infection early, when treatment can prevent progression to active disease.

Can TB spread through food or water?

TB bacteria in food or water can cause infection, but this is extremely rare in developed countries with safe water supplies and food handling practices. The vast majority of TB transmission happens through the air. Drinking unpasteurized milk from infected cattle is the main food-related route, and this is uncommon in the United States.