Tuberculosis (TB) is a bacterial infection that primarily attacks the lungs, destroying tissue from the inside out as the body’s own immune system wages an intense battle against it. In 2023, an estimated 10.8 million people fell ill with TB and 1.25 million died from it, making it the world’s leading infectious disease killer, surpassing even COVID-19. What makes TB unusual is that the bacteria can live inside your body for years or decades without causing symptoms, then suddenly activate and begin causing serious damage.
How TB Gets Into Your Lungs
TB spreads through the air. When someone with active TB coughs, they release tiny droplets containing the bacterium Mycobacterium tuberculosis. These droplets are small enough to stay airborne for hours. In lab conditions, TB bacteria in airborne droplets have remained viable after 12 hours of suspension. When you breathe them in, the particles travel deep into your lower airways.
Once there, the bacteria are swallowed by immune cells called alveolar macrophages, the lung’s first line of defense. This is where TB pulls its most important trick: instead of being destroyed inside these immune cells, the bacteria survive and multiply within them. The infected macrophages then migrate from the airway surface deeper into lung tissue, effectively carrying the infection further into the body. This early movement is what allows TB to establish itself before the broader immune system can respond.
The Granuloma: A Cage That Works Both Ways
Within weeks, your immune system recognizes the threat and mounts a larger response. More immune cells rush to the infection site, and they organize themselves into a tight, spherical structure called a granuloma. Infected macrophages sit at the center, surrounded by layers of different immune cells. Think of it as a living wall built to contain the bacteria.
This containment mostly works. The granuloma creates a microenvironment where the infection can be controlled, and in most people, the bacteria are trapped indefinitely. But “controlled” is not the same as “eliminated.” The bacteria persist in a dormant state inside the granuloma, sometimes for decades. Some of the immune cells within the granuloma actually lose their ability to kill bacteria over time, transforming into fat-filled cells that essentially give TB a safe place to hide. The granuloma is the body’s best available strategy, but it’s an imperfect one.
Latent vs. Active TB
This standoff between your immune system and the trapped bacteria is called latent TB. You carry the infection, but you feel fine, you’re not contagious, and you may never know you have it. Roughly 5% of otherwise healthy adults who become infected will develop active disease within the first two years. For those who don’t progress early, the annual risk of reactivation drops dramatically, to roughly 10 to 20 per 100,000 people per year.
The shift from latent to active TB happens when the immune system weakens. HIV is the most significant risk factor, but anything that suppresses immune function can tip the balance: malnutrition, diabetes, certain medications, aging, or other serious illness. When the immune system can no longer maintain the granuloma, the bacteria reactivate and begin multiplying again.
How TB Destroys Lung Tissue
Active TB causes damage through a process that is, paradoxically, driven by your own immune system. When the bacteria begin multiplying unchecked inside macrophages, specialized immune cells (cytolytic T lymphocytes) kill the infected macrophages to prevent further bacterial growth. The collateral cost is destruction of the surrounding lung tissue. The body is essentially sacrificing its own tissue to stop the bacteria from spreading.
This destruction produces a cheese-like dead tissue called caseum. In most cases, the bacteria trapped in this material can’t multiply because the environment lacks oxygen and has an acidic pH. But in up to 10% of infected people, the caseum softens. When softened material breaks through into an airway, it gets coughed out, leaving behind a hollow cavity in the lung. Oxygen floods into this cavity, and for the first time in the disease’s progression, the bacteria can multiply freely and in enormous numbers outside of cells. This is when TB becomes highly contagious and most dangerous.
Why TB Makes You Feel So Sick
The hallmark symptoms of active TB are a persistent cough (often with blood), fever, drenching night sweats, and significant weight loss. These aren’t caused by the bacteria directly damaging tissue. They’re caused by your immune system’s inflammatory response.
The immune battle against TB triggers a flood of inflammatory signaling molecules, particularly TNF-alpha. This molecule acts directly on muscle cells, accelerating protein breakdown and inhibiting the production of new muscle protein. The result is progressive muscle wasting, weakness, and fatigue. TB-related weight loss can be severe enough to resemble starvation, which is why the disease was historically called “consumption.”
Fever occurs in 60% to 85% of all TB cases and results from the inflammatory cascade itself, along with toxic byproducts absorbed from damaged lung tissue and cavities. Night sweats follow the same pattern, driven by the body’s attempts to regulate temperature during intense immune activity.
TB Beyond the Lungs
Though TB is best known as a lung disease, the bacteria can spread through the bloodstream to nearly any organ. The symptoms depend on where the infection lands:
- Kidneys: blood in the urine
- Brain and spinal membranes (TB meningitis): headache, confusion
- Spine: back pain
- Larynx: hoarseness
- Lymph nodes: swollen glands, often in the neck
- Bones and joints: swelling and pain in the affected joint
Regardless of which organ is affected, TB tends to produce the same core systemic symptoms: fever, night sweats, weight loss, loss of appetite, and a general sense of illness and low energy. When TB spreads widely through the bloodstream and seeds multiple organs at once, it’s called miliary TB, one of the most serious forms of the disease.
How TB Is Detected
Diagnosing TB has historically been slow. The gold standard, growing the bacteria from a sputum sample in a lab culture, can take up to six weeks because the organism grows so slowly. Newer molecular tests have dramatically changed this. The GeneXpert test, which detects TB DNA directly from a sample, delivers results in under two hours with 96% sensitivity overall and 100% sensitivity for respiratory samples. It can also detect drug resistance in the same test, which is critical for choosing the right treatment.
For latent TB, which produces no symptoms and no bacteria in sputum, diagnosis relies on skin tests or blood tests that measure your immune system’s memory of prior TB exposure. These tests can’t distinguish between latent infection and past infection that’s already been cleared, which sometimes complicates interpretation.
What Treatment Looks Like
Active TB is treated with a combination of antibiotics taken over a minimum of six months. The long duration is necessary because dormant bacteria inside granulomas are much harder to kill than actively growing ones. Most people start feeling better within a few weeks and stop being contagious relatively quickly after starting treatment, but stopping early is one of the main drivers of drug-resistant TB.
Latent TB can also be treated, typically with a shorter course of medication, to prevent future reactivation. This is especially important for people with weakened immune systems, where the risk of progression is substantially higher. The lung damage from cavities and tissue destruction can be permanent even after the infection is cured, and some people are left with reduced lung function, scarring, or chronic breathing difficulties.