Pulmonary tuberculosis is a bacterial infection of the lungs caused by Mycobacterium tuberculosis. It remains one of the deadliest infectious diseases on the planet, responsible for an estimated 1.25 million deaths and 10.8 million new cases worldwide in a single year. The infection spreads through the air when someone with active disease coughs, sneezes, or even sings, and it can linger in the body for years without causing any symptoms.
How TB Infects the Lungs
When you breathe in TB bacteria, they travel deep into the lungs and land in the tiny air sacs called alveoli. Immune cells called macrophages rush to the site and swallow the bacteria, which is normally how the body destroys invaders. But TB bacteria have evolved a workaround: they block the internal killing process inside macrophages and actually replicate within these cells. Eventually the bacteria destroy the host cell, break free, and spread into surrounding lung tissue.
In most healthy people, the immune system walls off the bacteria into small clusters and keeps them contained. This is latent TB infection. The bacteria are alive but dormant, and you won’t feel sick or be contagious. Roughly 5 to 10 percent of people with latent TB will progress to active disease at some point in their lives, often when their immune system weakens.
Latent TB vs. Active TB Disease
This distinction matters because it determines whether you’re contagious and whether you need treatment now or preventively. With latent TB, you have a small number of dormant bacteria in your body. You feel completely normal, have no symptoms, and cannot spread TB to anyone. A skin test or blood test will come back positive, but a chest X-ray typically looks normal.
Active TB disease is a different situation. The bacteria are multiplying, damaging lung tissue, and often spreading through the air when you cough. You’ll feel sick, lose weight, and may have visible changes on a chest X-ray, including cavities in the lung tissue. Active TB requires immediate treatment both for your health and to protect the people around you.
Symptoms of Active Pulmonary TB
The hallmark symptom is a persistent cough lasting three weeks or longer. It often produces thick sputum and, in some cases, blood. Chest pain is common, particularly with deep breathing or coughing.
Beyond the lungs, active TB causes systemic symptoms that develop gradually over weeks:
- Drenching night sweats that soak through clothing or bedding
- Unexplained weight loss and loss of appetite
- Persistent low-grade fever and chills
- Fatigue and weakness that worsens over time
These symptoms tend to creep in slowly, which is one reason TB often goes undiagnosed for weeks or months. The gradual onset can look like a lingering cold or flu until the weight loss and night sweats become hard to ignore.
How TB Spreads
TB travels through tiny airborne particles called droplet nuclei. When someone with active pulmonary TB coughs, these particles can remain suspended in the air for several hours, especially in enclosed, poorly ventilated spaces. Sneezing and singing also release the bacteria.
Not everyone exposed to TB will get infected. The risk depends on how many bacteria the sick person is shedding (people with lung cavities visible on X-ray shed the most), how close you are to them, and how long you spend in the same space. Among people who live in the same household as a TB patient, roughly 30 percent become infected. Once someone with active TB starts effective treatment, their ability to spread the disease drops dramatically within days to weeks.
Who Is Most at Risk
Anyone can catch TB, but certain groups face a much higher risk of both exposure and progression to active disease. HIV infection is the single biggest risk factor because it severely weakens the immune cells TB targets. Other conditions that raise your risk include diabetes, severe kidney disease, head and neck cancer, silicosis (a lung disease from inhaling silica dust), and low body weight. Medications that suppress the immune system, such as corticosteroids, treatments for rheumatoid arthritis or Crohn’s disease, and anti-rejection drugs after organ transplants, also increase vulnerability. Children under five are particularly susceptible because their immune systems are still developing.
Exposure risk is highest for people who were born in or frequently travel to countries in Asia, Africa, and Latin America where TB is common. Living or working in crowded settings like homeless shelters, prisons, nursing homes, and hospitals also increases the chance of encountering the bacteria. Injection drug use is another independent risk factor.
How Pulmonary TB Is Diagnosed
Diagnosing TB usually involves several tests used together, because no single test is definitive on its own.
A chest X-ray is typically the first step. It can reveal abnormal spots, dense areas, or cavities in the lungs that suggest TB. However, these findings can also result from other conditions, so an X-ray alone cannot confirm TB. On the flip side, a normal chest X-ray in someone with no symptoms generally rules out active pulmonary disease.
Sputum smear microscopy is the traditional lab test. You cough up a deep sample of phlegm, which is smeared on a glass slide, stained, and examined under a microscope for the bacteria. The number of bacteria visible on the slide is graded on a scale from 1+ to 4+, and a higher count indicates greater infectiousness. A negative smear, however, does not rule out TB, since people with lower bacterial loads can still have active disease.
A newer molecular test called the Xpert MTB/RIF assay has become widely used. A sputum sample is placed into a specialized machine that can detect TB DNA and simultaneously check for resistance to rifampin, one of the most important TB drugs. Results come back in about two hours, compared to weeks for traditional culture methods. Despite its speed, this test does not replace the need for a full bacterial culture, which remains the gold standard for confirming TB and testing sensitivity to the full range of antibiotics.
Treatment: What to Expect
Standard treatment for pulmonary TB uses a combination of four antibiotics and lasts six to nine months. It’s split into two phases. The intensive phase covers the first two months, during which you take all four drugs daily (or five days a week). This aggressive start kills the vast majority of bacteria quickly, and most people stop being contagious within a few weeks. The continuation phase lasts another four to seven months, using two of the original four drugs to eliminate the remaining bacteria hiding in the body.
Completing the full course is critical. Stopping early, even if you feel better, allows surviving bacteria to regrow and potentially develop resistance. This is how multidrug-resistant TB (MDR-TB) emerges. MDR-TB is defined as TB that resists at least the two most effective first-line drugs. It requires longer, more complex treatment with less effective medications and worse outcomes. Globally, about 400,000 new MDR-TB cases and 150,000 deaths from drug-resistant strains occur each year.
To help patients stick with treatment, many programs use directly observed therapy, where a healthcare worker watches you take each dose. This approach significantly improves cure rates and reduces the development of resistance.
The BCG Vaccine and Its Limits
The BCG vaccine, developed over a century ago, is the only TB vaccine in use. It’s given to infants in many countries where TB is common and is effective at preventing severe forms of childhood TB, including TB meningitis. Its performance against adult pulmonary TB, however, is inconsistent, with efficacy estimates ranging from zero to 80 percent depending on the geographic region.
This variability has puzzled researchers for decades. One leading explanation is that people living in tropical regions have more exposure to environmental mycobacteria (related bacteria found naturally in soil and water), which may interfere with the vaccine’s ability to train the immune system. Importantly, the same BCG strains that fail against adult lung TB in one country can work well in another, ruling out manufacturing quality as the main issue. Because of this unreliable protection, BCG is not a substitute for infection control, early diagnosis, and full treatment completion.