Tuberculosis (TB) is caused by a bacterium called Mycobacterium tuberculosis. This single species is responsible for nearly all human cases of the disease, which in 2023 reclaimed its position as the world’s leading infectious disease killer, surpassing COVID-19. The bacterium spreads through the air when someone with active TB coughs, speaks, or sings, and the tiny particles carrying it can linger in indoor air for several hours.
The Bacterium Behind TB
Mycobacterium tuberculosis is an unusually tough organism. It divides slowly, taking 12 to 24 hours to double its numbers under ideal conditions. Most common bacteria can do the same in 20 minutes. That slow growth is one reason TB infections develop gradually and why treatment takes months rather than days.
What really sets this bacterium apart is its cell wall. It has an exceptionally thick, waxy outer layer that acts like armor, blocking many of the drugs and immune chemicals the body throws at it. This barrier is a major reason the bacterium is so difficult to kill and why TB requires a combination of medications taken over a long period.
How It Spreads From Person to Person
TB is an airborne infection. When a person with active lung or throat TB coughs, talks, or sings, they release microscopic droplets containing the bacteria. These droplets are small enough to float in the air and be inhaled deep into the lungs of anyone nearby. Unlike infections spread by touch or contaminated surfaces, TB travels almost exclusively through shared air.
Transmission is tightly linked to indoor environments. Crowded, poorly ventilated spaces like hospitals, prisons, refugee camps, and even public buses create ideal conditions for the bacteria to pass between people. Outdoors, air dilution makes transmission far less likely. TB has been called the world’s most important building-associated illness for exactly this reason: the same infectious person will spread the disease to far more people in a sealed, stuffy room than in an open-air setting.
What Happens Inside Your Lungs
Once inhaled, the bacteria land in the small air sacs of the lungs, where immune cells called macrophages swallow them. Normally, macrophages destroy invaders. Mycobacterium tuberculosis has evolved to survive inside these cells, essentially hiding within the very immune defenses meant to kill it. Within minutes of being engulfed, the bacterium begins switching on genes that help it tolerate the acidic, low-nutrient environment inside the macrophage.
The immune system responds by walling off the infected cells in clusters of tissue called granulomas. Inside these structures, the bacteria face low oxygen and scarce nutrients. Rather than dying, they shift into a dormant state, slowing their metabolism and essentially going to sleep. This dormancy is the biological basis of what doctors call latent TB infection, a state where the bacteria are alive but not causing symptoms or spreading to other people.
Latent TB vs. Active TB
Most people who breathe in the bacteria don’t get sick right away. Their immune system contains the infection, and they carry the bacteria without symptoms, often without ever knowing it. This is latent TB. People with latent TB are not contagious and feel perfectly healthy.
Only about 5 to 15 percent of people with latent TB will ever develop active disease, typically within the first two to five years after infection. Active TB is the stage that causes illness: persistent cough, weight loss, fever, night sweats, and sometimes coughing up blood. It’s also the only stage where a person can spread the bacteria to others. The distinction matters because treating latent TB can prevent it from ever becoming active and contagious.
Who Is Most at Risk
Anything that weakens the immune system raises the chance that latent TB will progress to active disease. HIV is the most significant risk factor because it directly attacks the immune cells responsible for keeping TB bacteria contained. People living with HIV are many times more likely to develop active TB than the general population.
Diabetes is another major driver. In 2023, it was the most commonly reported medical risk factor among people diagnosed with active TB in the United States. People with diabetes have a harder time mounting an effective immune response, which gives dormant bacteria a window to reactivate. Smoking, malnutrition, heavy alcohol use, and medications that suppress the immune system (such as those used after organ transplants or for autoimmune diseases) also increase risk substantially.
Environmental conditions play an equally important role. Living or working in overcrowded housing, spending time in poorly ventilated institutional settings, or being incarcerated all raise the likelihood of both initial exposure and repeated reinfection. The combination of a weakened host and a crowded, sealed environment is where TB thrives most.
TB Beyond the Lungs
While the lungs are the primary target, Mycobacterium tuberculosis can spread through the bloodstream to virtually any organ. The most commonly affected sites outside the lungs include the lining around the lungs (pleura), lymph nodes, the gastrointestinal tract, bones and joints, the central nervous system, and the urinary and reproductive organs. TB that occurs outside the lungs is generally not contagious, but it can be harder to diagnose because symptoms vary widely depending on the organ involved.
How Drug Resistance Develops
The bacterium’s slow growth and protective cell wall already make it hard to treat. When patients don’t complete their full course of treatment, or when drug levels in the body are too low, surviving bacteria with naturally occurring genetic mutations gain a selective advantage. These resistant bacteria multiply and can be transmitted to new people, who then have a drug-resistant infection from the start.
Multidrug-resistant TB (MDR-TB) refers to strains that no longer respond to the two most effective first-line antibiotics. Globally, these strains account for hundreds of thousands of cases each year. Even more dangerous are extensively drug-resistant strains (XDR-TB), which resist additional backup medications and have been reported in over 100 countries. Drug resistance doesn’t change how TB spreads, but it makes treatment far longer, more complex, and less likely to succeed.
How TB Is Diagnosed
If your doctor suspects TB, you’ll typically go through a combination of tests. A TB skin test or a blood test can reveal whether you’ve been exposed to the bacteria, but neither can distinguish latent infection from active disease. A chest X-ray helps identify lung damage or active infection. The definitive confirmation comes from collecting a sputum sample (mucus coughed up from the lungs) and growing the bacteria in a lab culture. Newer molecular tests can detect TB DNA in sputum within hours, which is much faster than traditional cultures that can take weeks. Drug susceptibility testing then determines which medications will be effective against the specific strain.