How tuberculosis infection starts

Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis. These bacteria spread through the air when an infected person coughs, sneezes, speaks, or sings. If you breathe in these bacteria, they can lodge in your lungs and begin to multiply. Not everyone who breathes in TB bacteria becomes sick — your immune system may stop the infection before symptoms appear.

The bacteria prefer to grow in the lungs, but they can travel through your bloodstream to other parts of your body, including the kidneys, spine, brain, and lymph nodes. This is why TB can affect different organs depending on where the infection takes hold.

Key Takeaways

  • TB spreads through airborne bacteria when an infected person coughs or sneezes, not through touching or sharing food.
  • Your immune system can stop TB bacteria before you become sick, which is called latent TB infection.
  • Active TB disease develops when your immune system cannot control the bacteria, allowing them to multiply and damage lung tissue.
  • Certain conditions — like HIV, diabetes, or malnutrition — weaken your immune system and increase the risk that latent TB will become active.
  • TB bacteria can survive in your body for years without causing symptoms, but can reactivate if your immunity drops.

The difference between latent and active TB

When TB bacteria enter your lungs, one of two things happens. Your immune system may wall off the bacteria so they cannot spread or cause damage. You have no symptoms, feel fine, and cannot spread TB to others. This is called latent TB infection. A skin test or blood test will show you have been exposed, but you are not sick.

In other cases, your immune system cannot contain the bacteria. They multiply rapidly, damage lung tissue, and cause symptoms like cough, fever, and chest pain. This is active TB disease. People with active TB can spread the infection to others through the air. Without treatment, active TB can be life-threatening.

The difference matters because latent TB will not make you sick and you cannot pass it to others, but it can turn into active TB later if your immunity weakens. Active TB requires immediate treatment to stop the bacteria and prevent spread.

Why some people develop active TB and others do not

Whether latent TB stays dormant or becomes active depends on your immune system's strength. When you are healthy and your immune system is strong, it can keep TB bacteria under control for years or even your whole life. But if something weakens your immunity, the bacteria can break free and multiply.

Several conditions increase the risk that latent TB will become active. HIV infection is the strongest risk factor — if your CD4 count drops below 200, your risk of active TB rises sharply. Diabetes makes it harder for your immune system to fight infection. Kidney disease, malnutrition, and severe stress can also lower immunity. Certain medications that suppress the immune system, such as those used to treat rheumatoid arthritis or Crohn's disease, increase TB risk as well.

Age matters too. Young children under five and older adults over 65 have weaker immune systems and are more likely to develop active TB after infection. People who were recently infected with TB bacteria — within the past two years — are at higher risk than those infected long ago.

How TB bacteria damage your lungs

When TB bacteria multiply in your lungs, they trigger inflammation. Your body's immune cells rush to the infected area and try to destroy the bacteria, but in the process they damage healthy lung tissue. Over time, this creates cavities — hollow spaces — in the lungs where bacteria thrive and multiply even faster.

The damaged tissue cannot exchange oxygen properly, which is why people with active TB develop a persistent cough and shortness of breath. As the disease progresses without treatment, more lung tissue is destroyed. The bacteria can also spread to the airways and cause you to cough up blood. If TB spreads to other organs, it can damage the kidneys, spine, brain, or heart depending on where the infection takes hold.

Risk factors that increase your chance of TB infection

Some people are more likely to be exposed to TB bacteria in the first place. If you live or work in close quarters with someone who has active TB — such as in a crowded shelter, prison, or healthcare facility — your exposure risk is high. Healthcare workers, prison staff, and people who care for TB patients face repeated exposure.

Homelessness, poverty, and overcrowded living conditions increase both exposure and the risk that infection will progress to active disease. Substance use disorder can weaken immunity and make it harder to complete TB treatment. Recent immigration from countries where TB is common also raises the likelihood of exposure, since TB rates are much higher in some parts of the world than others.

Why TB bacteria are difficult to kill

TB bacteria have a thick, waxy outer coating that protects them from many antibiotics and from your immune system's attack. This coating allows them to survive inside immune cells that would normally destroy other bacteria. The bacteria also grow slowly compared to other infections, which means TB treatment takes months rather than weeks.

Because TB bacteria are so hardy, they can survive in your body for years in a dormant state. They can also develop resistance to antibiotics if treatment is incomplete or interrupted. This is why TB treatment must continue for the full course — usually six months or longer — even after you feel better. Stopping early allows bacteria to survive and can lead to drug-resistant TB, which is much harder to treat.

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 shaking hands, sharing utensils, or touching an infected person's skin. You need to breathe in the bacteria for infection to occur, which usually requires close, prolonged contact with someone who has active TB disease.

If I have latent TB, will it always turn into active TB?

No. Most people with latent TB never develop active disease. Your immune system can keep the bacteria dormant for your entire life. Active TB develops in only about 5 to 10 percent of people with latent infection, usually when something weakens their immunity. If you have latent TB and a condition that weakens immunity — like HIV — your doctor may recommend preventive treatment to lower your risk.

How long can TB bacteria live in your body?

TB bacteria can survive in your body for years or decades without causing symptoms. This is why latent TB can reactivate years after the initial infection, even if you felt fine in the meantime. The bacteria remain dormant, walled off by your immune system, until something changes that allows them to multiply again.

Does everyone exposed to TB bacteria get infected?

No. Infection depends on how much bacteria you breathe in and how strong your immune system is. A single brief exposure is unlikely to cause infection. People who live or work closely with someone who has active TB over weeks or months are at much higher risk. Your immune system may also fight off a small exposure before infection takes hold.