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

Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis. A person with active TB disease in the lungs or throat releases these bacteria into the air in tiny droplets when they cough, sneeze, speak, or sing. Another person breathes in those droplets and the bacteria settle in their lungs. This is the only way TB spreads—you cannot get it from touching someone, sharing food, or shaking hands.

Not everyone who breathes in TB bacteria gets sick. Whether you develop active TB disease depends on your immune system's strength and how much bacteria you were exposed to. Most people who inhale TB bacteria develop latent TB infection instead, meaning the bacteria are in their body but dormant and cause no symptoms. Only about 5 to 10 percent of people with latent TB ever progress to active disease.

Key Takeaways

  • TB spreads only through the air when someone with active lung or throat TB coughs, sneezes, or speaks.
  • A weakened immune system is the main reason latent TB infection becomes active TB disease.
  • HIV, certain medications, malnutrition, and chronic diseases like diabetes all increase the risk that dormant TB will activate.
  • Close, prolonged contact with someone who has active TB—such as living in the same household—raises your risk of infection.
  • TB is more common in crowded settings with poor ventilation, where the bacteria can linger in the air longer.

A weakened immune system is the primary reason TB becomes active

Your immune system normally keeps TB bacteria in check. When your immune system is weakened, the bacteria can multiply and cause active TB disease. HIV is the strongest risk factor—people with HIV are 20 to 30 times more likely to develop active TB than people without HIV. If your CD4 count (a measure of immune cells) drops below 200, your risk becomes very high.

Other conditions that weaken immunity also increase TB risk. These include diabetes, chronic kidney disease, cancer, and severe malnutrition. Certain medications that suppress the immune system—such as TNF-alpha inhibitors used for rheumatoid arthritis or Crohn's disease—also raise the risk. Even stress and poor sleep can gradually weaken immune function over time.

Age and recent infection affect whether TB develops

Children under 5 years old and adults over 65 have weaker immune responses and are at higher risk for active TB. Infants and very young children are especially vulnerable to severe forms of TB that spread beyond the lungs.

People who were recently infected with TB bacteria—within the past 2 years—are more likely to develop active disease than those infected long ago. This is why people who have just been exposed to someone with active TB are often given preventive medication, even if they have no symptoms yet.

Close contact and crowded living conditions increase exposure risk

Living in the same household as someone with active TB disease puts you at much higher risk of breathing in the bacteria. Healthcare workers, prison staff, and shelter workers who spend hours near people with TB also face higher exposure. The longer and closer the contact, the greater the chance of infection.

TB spreads more easily in crowded places with poor ventilation—homeless shelters, prisons, nursing homes, and poorly ventilated housing. In these settings, bacteria can remain suspended in the air for hours, and many people may breathe the same contaminated air. Overcrowding alone does not cause TB, but it makes transmission much more likely when someone with active disease is present.

Substance use and smoking damage lung defenses

Smoking and heavy alcohol use both weaken the lungs' ability to fight infection. Smoking damages the cells that line the airways and clear bacteria. Alcohol weakens immune function throughout the body. People who smoke or drink heavily are at higher risk of developing active TB if they have been exposed to the bacteria.

Injection drug use carries additional risk because it can introduce bacteria directly into the bloodstream, and people who inject drugs often have poor nutrition and weakened immunity. Crack cocaine use also damages lung tissue and increases TB risk.

Certain medical treatments increase TB risk

Medications that suppress the immune system—used to treat autoimmune diseases, prevent organ rejection after transplant, or manage cancer—can allow dormant TB to reactivate. TNF-alpha inhibitors (used for rheumatoid arthritis, psoriasis, and inflammatory bowel disease) carry a particularly high risk. Immunosuppressive drugs used after organ transplant also increase TB risk significantly.

Corticosteroids, especially at high doses or for long periods, can weaken immunity enough to allow TB to develop. If you take any of these medications and have latent TB, your doctor may recommend preventive TB treatment to reduce the chance of reactivation.

Geography and TB prevalence in your community matter

TB is more common in some parts of the world than others. Countries with high TB rates, crowded urban areas, and regions with limited access to healthcare see more cases. If you live in or have recently traveled to a high-TB area, your risk of exposure is higher. Healthcare workers in hospitals that treat TB patients also face occupational exposure.

Within the United States, TB rates are higher in certain urban centers and among specific populations with limited healthcare access. However, TB can occur anywhere, and anyone exposed to someone with active disease can become infected regardless of location.

Frequently Asked Questions

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

No. People with latent TB have the bacteria in their body but are not sick and do not spread TB to others. Only people with active TB disease—usually in the lungs or throat—can transmit the bacteria through the air.

If I was exposed to TB, will I definitely get sick?

No. Most people who breathe in TB bacteria develop latent infection instead of active disease. Your immune system may keep the bacteria dormant for life. Active disease develops in only about 5 to 10 percent of people with latent TB, usually when immunity weakens.

Does TB run in families?

TB itself is not inherited, but family members of someone with active TB are at higher risk of exposure because they share the same living space. If multiple family members develop TB, it is because they were all exposed to the same infected person, not because of genetics.

Can I get TB from food or water?

TB spreads only through the air. You cannot get it from food, water, or surfaces. In rare cases, unpasteurized milk from infected cattle can carry a related TB bacterium, but this is extremely uncommon in countries with food safety regulations.

How long does someone with active TB spread the disease?

People with untreated active TB can spread the disease as long as they have symptoms. Once someone starts appropriate TB treatment, they usually become non-contagious within 2 to 3 weeks, though they may still have symptoms for longer.