Tuberculosis starts when you breathe in bacteria called Mycobacterium tuberculosis

Tuberculosis (TB) is caused by a single bacterium: Mycobacterium tuberculosis. You catch it by breathing in air droplets from someone with active TB disease — usually when they cough, sneeze, speak, or sing. The bacteria travel into your lungs, where they settle and begin to multiply. Not everyone who breathes in the bacteria develops TB disease; your immune system may stop the infection before symptoms appear.

The bacteria are slow-growing and can survive in the body for years without causing illness. This is why TB has two distinct forms: latent TB infection (where you carry the bacteria but feel well and cannot spread it) and TB disease (where the bacteria are actively multiplying and damaging lung tissue). Understanding which form you have matters because the treatment and risk of spreading differ significantly.

Key Takeaways

  • Mycobacterium tuberculosis bacteria spread through the air when a person with active TB coughs, sneezes, or speaks, and you inhale the droplets.
  • Your immune system can stop the infection at the latent stage, meaning you carry the bacteria but have no symptoms and cannot spread TB to others.
  • TB disease develops when your immune system fails to contain the bacteria, allowing them to multiply and damage lung tissue over weeks or months.
  • Certain conditions — HIV infection, diabetes, malnutrition, and immunosuppressive medications — significantly increase the risk that latent TB will progress to active disease.

How the bacteria enter and settle in your lungs

When someone with active TB releases bacteria into the air, the particles are tiny enough to reach the deepest parts of your lungs called the alveoli. These are the air sacs where oxygen normally passes into your bloodstream. The bacteria settle here and begin to divide slowly — much more slowly than most bacteria, which is one reason TB is hard to treat quickly.

Your immune system recognizes the bacteria as foreign and mounts a response. White blood cells called macrophages surround and try to kill the invaders. In many people, this response is strong enough to stop the bacteria from multiplying further. The bacteria become dormant, enclosed in a wall of immune cells, and you develop latent TB infection. You will have no symptoms, feel completely well, and cannot spread TB to anyone else.

Why some people develop active TB disease and others do not

Whether latent TB progresses to active disease depends almost entirely on your immune system's strength. If your immune system remains strong, the bacteria stay contained indefinitely — some people carry latent TB their entire lives without ever developing disease. But if your immune system weakens, the bacteria can break through the immune barrier, multiply rapidly, and cause active TB disease.

The risk of progression is highest in the first two years after infection, but it can happen at any point. A person with untreated latent TB has roughly a 5 to 10 percent lifetime risk of developing active disease, though this varies based on individual factors. Once active disease develops, the bacteria damage lung tissue, causing the symptoms of TB: persistent cough, chest pain, fever, and night sweats.

Conditions that weaken immunity and increase TB risk

Certain medical conditions make it much more likely that latent TB will progress to active disease. HIV infection is the strongest risk factor — people with HIV and untreated latent TB have a 10 percent annual risk of developing active TB, compared to the 5 to 10 percent lifetime risk in people with healthy immune systems. Uncontrolled diabetes also increases risk, as does severe malnutrition and kidney disease.

Medications that suppress the immune system — such as TNF-alpha inhibitors used for rheumatoid arthritis and Crohn's disease — can reactivate latent TB. Immunosuppressive drugs given after organ transplants carry similar risk. Even heavy alcohol use and smoking damage the lungs' ability to fight infection. Age matters too: very young children and people over 65 have weaker immune responses and higher progression risk.

Why TB bacteria are difficult for the body to eliminate

The bacteria that cause TB have evolved several survival strategies that make them hard for your immune system to kill. They have a thick, waxy outer coating that resists many antibiotics and makes it difficult for immune cells to penetrate. The bacteria can also hide inside macrophages — the very cells meant to destroy them — and survive there for years.

Additionally, Mycobacterium tuberculosis grows very slowly compared to other bacteria. This slow growth means your immune system has a harder time detecting and responding to the infection, and it also means antibiotic treatment must continue for months rather than days or weeks. The bacteria can also enter a dormant state where they consume very little energy and become even harder to kill with drugs.

The difference between latent and active TB at the bacterial level

In latent TB infection, the bacteria are alive but not multiplying significantly. They are enclosed in a granuloma — a structure formed by immune cells that walls off the infection. The bacteria remain in this dormant state, and your body maintains a balance: the immune system keeps them contained, but cannot fully eliminate them. This balance can last a lifetime if your immunity stays strong.

In active TB disease, this balance breaks down. The bacteria break free from the granuloma and begin multiplying rapidly. As they multiply, they damage the lung tissue around them, creating cavities and causing inflammation. This is when symptoms appear and when the disease becomes contagious — the active multiplication of bacteria produces the cough that spreads TB to others. The bacteria can also spread through the bloodstream to other organs, causing TB disease outside the lungs.

How TB spreads from person to person

TB spreads only from people with active disease in the lungs or throat — not from people with latent TB. When someone with active TB coughs or sneezes, they release droplets containing the bacteria into the air. These droplets are small enough to float for hours in an enclosed space. Anyone who breathes that air can inhale the bacteria.

Close, prolonged contact increases transmission risk. Family members, coworkers, and healthcare workers who spend hours near someone with untreated active TB are at higher risk than someone who has brief contact. The bacteria do not survive long in sunlight or fresh air, so TB spreads primarily indoors. Once someone with active TB begins antibiotic treatment, they usually become non-contagious within two weeks, even though the bacteria are not fully eliminated.

Frequently Asked Questions

Can you get TB from touching someone or sharing food with them?

No. TB spreads only through the air when someone with active lung or throat disease coughs, sneezes, or speaks. You cannot catch TB from touching skin, shaking hands, or sharing utensils, clothing, or bedding. The bacteria must reach your lungs to cause infection.

If I have latent TB, will I definitely develop active disease?

No. Most people with latent TB never develop active disease. Your risk depends on your immune system's strength and whether you have conditions like HIV, diabetes, or take immunosuppressive medications. Without these risk factors, your lifetime risk is roughly 5 to 10 percent.

How long does it take for TB disease to develop after breathing in the bacteria?

It varies. Some people develop active TB within weeks of infection, while others may not show symptoms for months or years. The progression depends on how quickly your immune system weakens and how many bacteria you were exposed to. People with HIV or other immune conditions typically progress faster.

Can TB bacteria survive outside the body?

TB bacteria can survive in dried sputum for several hours, but they die quickly in sunlight and fresh air. They do not survive well on surfaces or in food. The main transmission route is breathing in fresh air droplets from someone with active disease, not contact with contaminated objects.

Why does TB take so long to treat compared to other bacterial infections?

TB bacteria grow very slowly and have a thick, waxy coating that makes antibiotics less effective. The bacteria can also hide inside immune cells where drugs penetrate poorly. These characteristics mean treatment must continue for months — typically six months for drug-sensitive TB — rather than the days or weeks needed for other bacterial infections.