Tuberculosis is caused by bacteria that spread through the air when an infected person coughs, sneezes, or speaks

Tuberculosis (TB) happens when you breathe in bacteria called Mycobacterium tuberculosis. An infected person releases these bacteria into the air in tiny droplets when they cough or sneeze. You don't need prolonged contact—a few minutes in a poorly ventilated room can be enough. The bacteria land in your lungs and begin to multiply.

Not everyone who breathes in TB bacteria gets sick. Your immune system may kill the bacteria before it takes hold, or the infection may stay dormant in your lungs for years or decades. Whether you develop active TB depends on your immune strength and other factors in your health.

Key Takeaways

  • TB spreads through airborne droplets when an infected person coughs, sneezes, or speaks—not through touching, sharing food, or shaking hands.
  • You cannot catch TB from someone's blood, saliva, or sweat, and TB is not hereditary.
  • A weakened immune system—from HIV, malnutrition, diabetes, or certain medications—makes active TB much more likely after exposure.
  • Most people exposed to TB bacteria never develop the disease because their immune system controls the infection.
  • TB bacteria can lie dormant in your lungs for years, then reactivate later if your immunity drops.

How the bacteria enters your body and settles in the lungs

When you inhale air containing TB bacteria, the droplets travel down your windpipe into your lungs. The bacteria prefer the upper lobes of your lungs, where oxygen levels are highest. Once there, the bacteria begin to divide and damage lung tissue.

Your immune system responds by sending white blood cells to surround and contain the bacteria. In many cases, this works—your body walls off the infection in a scar-like structure called a granuloma, and the bacteria stay trapped and inactive. This is called latent TB infection. You have no symptoms, cannot spread TB to others, and may never know you were infected.

Why some people develop active TB and others don't

Whether latent TB becomes active TB depends mainly on your immune system's strength. If your immune system is strong, it keeps the bacteria contained indefinitely. If your immunity weakens, the bacteria can break free, multiply rapidly, and cause active TB disease.

The most common reason for immune weakness is HIV infection. People with HIV and untreated TB have a much higher risk of developing active disease. Other conditions that weaken immunity include severe malnutrition, uncontrolled diabetes, chronic kidney disease, and certain cancers. Some medications—particularly those that suppress the immune system for autoimmune diseases or after organ transplants—also increase risk.

Age matters too. Young children (under 5) and older adults have weaker immune responses and are at higher risk of active TB after exposure. People who were recently infected (within the past two years) are also more likely to develop active disease than those with older latent infections.

Conditions that make active TB more likely

Several health conditions significantly raise your risk of developing active TB if you carry the latent infection:

  • HIV/AIDS: The most powerful risk factor. Without treatment, people with HIV are 20 to 30 times more likely to develop active TB.
  • Diabetes: Uncontrolled blood sugar weakens immune response to TB bacteria.
  • Chronic kidney disease: Kidney failure reduces immune function and TB drug clearance from your body.
  • Malnutrition: Insufficient calories and protein impair immune cell production.
  • Alcohol use disorder: Heavy drinking damages lung tissue and weakens immunity.
  • Smoking: Damages lung tissue and reduces the lungs' ability to clear bacteria.

If you have any of these conditions and know you were exposed to TB, tell your doctor. Testing and preventive treatment can stop latent TB from becoming active.

Medications and treatments that increase TB risk

Certain drugs that suppress your immune system increase the risk that latent TB will reactivate. These include TNF-alpha inhibitors (used for rheumatoid arthritis and inflammatory bowel disease), some cancer treatments, and corticosteroids at high doses or for long periods.

If you take any immunosuppressive medication and have latent TB, your doctor may recommend preventive TB treatment to reduce the chance of reactivation. This is especially important before starting a new immune-suppressing drug.

Environmental and social factors that increase exposure risk

Some settings and situations put you in closer contact with people who have active TB. Healthcare workers, prison staff, and people living in crowded shelters face higher exposure. Poorly ventilated spaces—old buildings, basements, rooms without windows or air circulation—allow TB bacteria to linger longer in the air.

Living with someone who has untreated active TB is the highest-risk exposure. Close contacts of TB patients should be tested and may need preventive treatment even if they have no symptoms.

What TB bacteria need to survive and spread

TB bacteria thrive in warm, moist environments with good oxygen supply—which is why they prefer the upper lungs. They grow slowly compared to other bacteria, which is why TB disease develops gradually and why treatment takes months rather than weeks.

The bacteria are tough. They have a waxy outer coating that protects them from many antibiotics and from your immune system's first-line defenses. This is why TB requires specific drugs and why completing the full course of treatment is critical—stopping early allows resistant bacteria to survive and multiply.

Frequently Asked Questions

Can you catch TB from touching someone or sharing food?

No. TB spreads only through the air when an infected person coughs, sneezes, or speaks. You cannot catch it from touching skin, shaking hands, sharing utensils, or sharing a bed. TB bacteria cannot survive on surfaces or in saliva.

Is tuberculosis hereditary or passed down in families?

TB is not hereditary. You cannot inherit it from a parent. However, if a family member has active TB and lives in your home, you may be exposed to the bacteria through the air—that is exposure, not inheritance.

Can you get TB from someone with latent TB?

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

How long does it take to develop active TB after exposure?

This varies widely. Some people develop active TB within weeks or months of exposure. Others carry latent TB for years or decades before it reactivates. There is no fixed timeline—it depends on your immune system and overall health.

If I have latent TB, will I definitely get active TB later?

No. Most people with latent TB never develop active disease. About 5 to 10 percent of people with latent TB will develop active TB at some point in their life, usually within the first two years after infection. The risk is much higher if your immune system becomes weakened.