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 through the air and land in your lungs when you breathe them in.

You cannot get TB from shaking hands, sharing food or drink, touching someone's clothes, or sitting on the same toilet seat. The bacteria must reach your lungs to cause infection. This is why TB spreads most easily in enclosed spaces where people spend time together—homes, workplaces, schools, or shelters—especially if the air does not circulate well.

Not everyone who breathes in TB bacteria becomes sick. Your immune system may fight off the infection before it takes hold. This is why some people develop latent TB infection (the bacteria are in their body but not causing illness) while others develop active TB disease.

Key Takeaways

  • TB bacteria spread through the air when a person with active TB disease coughs, sneezes, or speaks, and you breathe in the bacteria.
  • You cannot catch TB through casual contact like handshakes, shared meals, or touching clothing.
  • Close, prolonged contact with someone who has active TB disease increases your risk of infection.
  • People with latent TB infection (bacteria in their body but no symptoms) cannot spread TB to others.
  • Your immune system may fight off TB bacteria after you breathe them in, preventing you from becoming sick.

Who is most likely to get TB after exposure

Certain groups of people have a higher risk of developing active TB disease after breathing in the bacteria. People with weakened immune systems are at greatest risk, including those with HIV, those taking medications that suppress immunity, and people with diabetes or kidney disease. Older adults and very young children (under 5 years old) also have weaker immune responses and are more vulnerable.

People who live or work closely with someone who has active TB disease face higher exposure. This includes family members, healthcare workers, and people in congregate settings like homeless shelters, prisons, or nursing homes. People who recently arrived in the United States from countries where TB is common also have higher rates of infection.

Substance use, malnutrition, and poverty increase risk as well, partly because they weaken the immune system and partly because they make close living conditions more likely. Someone who was infected with TB bacteria years ago but never developed symptoms is also at risk if their immune system weakens later in life.

The difference between latent and active TB infection

When you first breathe in TB bacteria, one of two things happens. Your immune system may stop the bacteria from growing, trapping them in your lungs in a dormant state. This is called latent TB infection. You have the bacteria in your body, but you have no symptoms, you do not feel sick, and you cannot spread TB to other people. A skin test or blood test will show that you have been infected, but a chest X-ray looks normal.

In other cases, the bacteria overcome your immune system and begin to multiply. This is active TB disease. You develop symptoms like a persistent cough, chest pain, fever, and night sweats. You can spread the disease to others. A chest X-ray shows signs of infection in your lungs, and tests of your sputum (the fluid you cough up) will show TB bacteria.

About 5 to 10 percent of people with latent TB infection will develop active disease at some point in their lives. The risk is highest in the first two years after infection but can happen decades later if the immune system weakens.

How long you are contagious if you have active TB

A person with active TB disease can spread the infection to others as long as the bacteria are alive in their lungs and airways. Before treatment begins, someone with active TB is most contagious. Within two weeks of starting the right antibiotics, most people become much less contagious, though they may still spread the disease if they cough or sneeze.

After about two to three weeks of treatment with standard TB medications, most people stop being contagious altogether. However, they must continue taking medications for the full course—usually six months—to kill all the bacteria and prevent the disease from returning. Stopping treatment early allows the bacteria to survive and multiply again, and it can create drug-resistant TB that is harder to treat.

People with latent TB infection are never contagious, even if they never receive treatment. The bacteria are dormant and do not travel through the air.

Settings where TB spreads most easily

TB spreads most readily in places where people spend many hours together in poorly ventilated spaces. Homes are a common setting, especially if family members share a bedroom or spend long hours in the same room. Workplaces with poor air circulation—factories, offices, or shops—can be sites of transmission if someone with active TB works there.

Congregate settings like homeless shelters, prisons, nursing homes, and hospitals see higher rates of TB spread because people live or work in close quarters for extended periods. Schools and childcare centers can be sites of transmission, though this is less common than in other settings. Healthcare facilities must follow strict infection control procedures to prevent TB from spreading to patients and staff.

Airplanes, buses, and trains carry a lower risk because air circulates more frequently, though prolonged close contact with someone who has active TB on a long flight or train ride can still lead to infection.

What happens after you breathe in TB bacteria

When you inhale TB bacteria, they land in the tiny air sacs of your lungs called alveoli. Your immune system immediately begins to respond. White blood cells surround the bacteria and try to kill them. In most people, this immune response is successful, and the bacteria are contained within a few weeks.

If your immune system is strong enough to stop the bacteria but not kill them completely, the bacteria become dormant. Your body walls them off in scar tissue, creating what looks like a small spot on a chest X-ray. This is latent TB infection. You may never know you have it unless a skin test or blood test reveals it.

If your immune system cannot contain the bacteria, they continue to multiply in your lungs. Over weeks or months, you develop symptoms of active TB disease. The bacteria damage lung tissue, causing the cough, fever, and other signs of illness. At this point, you can spread TB to others.

Risk factors that make TB infection more likely to progress to active disease

Several factors determine whether someone with latent TB infection will develop active disease. HIV infection is the strongest risk factor—people with HIV are about 20 times more likely to develop active TB than people without HIV. Other conditions that weaken immunity increase risk as well, including diabetes, chronic kidney disease, and cancer.

Medications that suppress the immune system, such as those used to treat rheumatoid arthritis or to prevent organ rejection after transplant, significantly raise the risk of active TB. Malnutrition and low body weight also weaken the immune response. Smoking and heavy alcohol use damage the lungs and immune system, making active disease more likely.

Age matters too. People over 65 and children under 5 have weaker immune systems and are at higher risk. People who were infected with TB bacteria within the past two years are at highest risk of developing active disease, which is why doctors sometimes recommend preventive treatment for recently infected people.

Frequently Asked Questions

Can you get TB from someone who has latent TB infection?

No. People with latent TB infection have bacteria in their body, but the bacteria are dormant and do not travel through the air. Only people with active TB disease can spread the infection to others.

How many times do you need to be exposed to TB to get infected?

There is no set number. Infection depends on how much bacteria you breathe in and how strong your immune system is. A single exposure to someone with active TB disease can lead to infection, especially if you spend many hours together in a closed space.

If someone in my house has active TB, will I definitely get it?

Not necessarily. Your risk is higher than the general population, but whether you become infected depends on how much time you spend near the person, how well the air circulates, and how strong your immune system is. Close family members have the highest risk.

Can TB spread through food or water?

TB bacteria spread through the air, not through food or water. You cannot get TB from eating or drinking after someone with TB, or from sharing utensils or cups.

How soon after exposure do symptoms appear?

If you develop active TB disease, symptoms usually appear within the first two years after infection, though they can develop decades later. Some people never develop symptoms and remain in the latent stage for life.