Mycobacterium tuberculosis is the bacterium that causes tuberculosis (TB), one of the deadliest infectious diseases in human history. In 2023, it killed an estimated 1.32 million people worldwide and newly infected 8.2 million, the highest number recorded since the World Health Organization began tracking TB in 1995. Despite being a preventable and treatable disease, TB remains a massive global health burden, largely because of the bacterium’s unusual biology and its ability to hide inside the human body for years.
What Makes This Bacterium Unusual
Mycobacterium tuberculosis has one of the most distinctive cell walls of any known bacterium. While it’s technically classified as a type of Gram-positive bacterium, it doesn’t behave like one under a microscope. Its cell wall is built from a rigid skeleton of sugars and proteins coated in mycolic acids, which are extremely long fatty acid chains that can stretch up to 100 carbon atoms. These waxy molecules form an outer membrane (sometimes called the mycomembrane) that acts almost like a suit of armor.
This thick, waxy coating is the reason TB bacteria are so hard to kill. It blocks many antibiotics from penetrating the cell, helps the bacterium survive inside immune cells, and makes it resistant to drying out. It’s also the reason TB bacteria are called “acid-fast”: once stained in a lab, the dye can’t be washed away with acid, which is one of the oldest and simplest ways to identify them under a microscope.
How TB Spreads
TB is an airborne disease. When someone with active TB in their lungs coughs, speaks, or sings, they release tiny particles called droplet nuclei that contain live bacteria. These particles are small enough to remain suspended in the air for several hours depending on ventilation and sunlight. A single inhaled droplet nucleus can be enough to start an infection.
This is what makes TB different from many other bacterial infections. You can’t catch it from touching surfaces, sharing utensils, or shaking hands. Prolonged close contact in enclosed spaces, like living in the same household, is the most common way it spreads. Poor ventilation, crowded housing, and indoor environments with little sunlight all increase the risk.
Latent TB vs. Active TB
Most people who breathe in TB bacteria don’t get sick right away. In about 90% of cases, the immune system manages to contain the infection without ever producing symptoms. This is called latent TB infection. The bacteria are alive inside the body but walled off and dormant. People with latent TB can’t spread it to others and feel perfectly healthy.
The risk is that latent TB can reactivate. About 5% of people with latent TB develop active disease within the first two years of infection, and roughly 10% will develop it at some point during their lifetime. The risk climbs significantly in people with weakened immune systems, particularly those living with HIV, people on immunosuppressive medications, and those with diabetes or malnutrition.
How the Bacteria Survive Inside the Body
When TB bacteria reach the lungs, immune cells called macrophages are the first to respond. These cells swallow the bacteria, attempting to digest them the same way they’d destroy any other invader. But TB has evolved a remarkable set of escape tactics. The bacteria can block the normal digestive process inside the macrophage, preventing the cell from breaking them down. They can even punch holes through the membrane of their compartment using a specialized protein, escaping into the cell’s interior where they multiply freely.
As infected macrophages die and burst, they release bacteria that neighboring macrophages swallow, repeating the cycle. This triggers the immune system to build a structure called a granuloma: a dense ball of immune cells that walls off the infection site. In a well-organized granuloma, layers of specialized macrophages pack tightly together, surrounded by a ring of other immune cells and eventually a layer of scar tissue. This structure physically contains the bacteria and cuts them off from the rest of the body. When a granuloma holds, that’s latent TB. When it breaks down, bacteria escape into the lung tissue and sometimes into the bloodstream, and that’s when active disease begins.
Symptoms of Active TB
Active TB in the lungs produces a persistent cough that lasts three weeks or longer, often producing sputum or, in more advanced cases, blood. Chest pain is common. Beyond the lungs, the disease causes systemic symptoms: unexplained weight loss, loss of appetite, fatigue, fever, chills, and drenching night sweats.
These symptoms tend to come on gradually. Many people with early active TB assume they have a lingering cold or bronchitis, which is one reason the disease often goes undiagnosed for weeks or months. During that delay, they can unknowingly spread it to others. Though TB most commonly affects the lungs, it can also infect the kidneys, spine, brain, and other organs.
How TB Is Diagnosed
There are two main screening methods. The tuberculin skin test involves injecting a small amount of protein under the skin of the forearm and checking for a raised bump 48 to 72 hours later. A blood test called an interferon-gamma release assay (IGRA) measures how your immune cells react to TB-specific proteins in a lab sample. Both tests detect whether your immune system has ever encountered TB bacteria, meaning they identify latent infection as well as active disease.
A positive result on either test doesn’t tell you whether the infection is latent or active. That requires further evaluation: a chest X-ray to look for lung damage and lab analysis of sputum samples to confirm whether live TB bacteria are present. The sputum culture remains the definitive way to diagnose active TB and determine which drugs the bacteria are susceptible to.
Treatment and Duration
Active TB requires a combination of four antibiotics taken during an initial intensive phase lasting about two months. After that, treatment continues with two of the drugs for an additional four to seven months, bringing the total course to six to nine months. Using multiple drugs simultaneously is essential because the bacteria mutate readily, and using a single antibiotic would quickly breed resistant strains.
The length of treatment is one of the biggest challenges in TB control. People often start feeling better within a few weeks and stop taking their medications early. Incomplete treatment is one of the primary drivers of drug-resistant TB.
Drug-Resistant TB
When TB bacteria develop resistance to the two most effective first-line antibiotics (isoniazid and rifampin), the result is multidrug-resistant TB, or MDR-TB. This form of the disease requires longer, more complex treatment regimens with drugs that tend to have harsher side effects. Extensively drug-resistant TB (XDR-TB) goes a step further, resisting those same two drugs plus key backup antibiotics, leaving very few treatment options. XDR-TB is rare, but outbreaks have occurred in parts of sub-Saharan Africa and Eastern Europe.
The BCG Vaccine
The BCG vaccine is the only widely used vaccine against TB. It is highly effective at protecting young children from the most dangerous forms of the disease: severe disseminated TB and TB meningitis. In newborns, vaccine efficacy against these life-threatening forms reaches about 90%. In school-age children who haven’t been previously exposed, protection is similarly high at around 92%.
The vaccine’s protection weakens substantially in older children and adults. For people over school age, efficacy against pulmonary TB drops below 20%. This is a major reason TB continues to spread so widely. The BCG vaccine can prevent children from dying of TB, but it does not reliably stop adolescents and adults from catching and transmitting the lung infection that drives the epidemic.