Tuberculosis (TB), once called consumption, is a highly infectious disease that primarily affects the lungs.

According to the , it’s one of the top 10 causes of death worldwide, killing 1.7 million people in 2016.

TB is most common in developing countries, but according to the , over 9,000 cases were reported in the United States in 2016.

Tuberculosis is usually preventable and curable under the right conditions.

Some people are infected with the TB bacteria but don’t experience symptoms. This condition is known as latent TB. TB can stay dormant for years before developing into active TB disease.

Active TB typically causes many symptoms that are most commonly related to the respiratory system, including coughing up blood or sputum (phlegm). You may experience a cough that lasts for over three weeks and pain when coughing or with normal breathing.

Other symptoms include:

While TB usually affects the lungs, it can also affect other organs, such as the kidneys, spine, bone marrow, and brain. Symptoms will vary depending on which organ is infected. For example, tuberculosis of the kidneys can cause you to urinate blood.

According to , more than 95 percent of all deaths related to TB cases occur in low- and middle-income countries.

People who use tobacco or misuse drugs or alcohol long term are more likely to get active TB, as are people diagnosed with HIV and other immune system issues. TB is the leading killer of people who are HIV-positive, according to . Other risk factors for getting active TB disease include:

Medications that suppress the immune system can also put people at risk for developing active TB disease, in particular medications that help prevent organ transplant rejection. Other medications that increase your risk of getting TB include those taken to treat:

Traveling to regions where TB rates are high also increases your risk of contracting the infection. These regions include:

  • sub-Saharan Africa
  • India
  • Mexico and other Latin American countries
  • China and many other Asian countries
  • parts of Russia and other countries of the former Soviet Union
  • islands of Southeast Asia
  • Micronesia

According to the , many low-income groups in the United States have limited access to resources needed to diagnose and treat TB, placing them at greater risk of active TB disease. People who are or have been homeless or in prison are at a higher risk of developing TB.

A bacteria called Mycobacterium tuberculosis causes TB. There are a variety of TB strains, and some have become resistant to medication.

TB bacteria are transmitted through infected droplets in the air. Once they’re in the air, another nearby person can inhale them. A person who has TB can be transmitted the bacteria via:

  • sneezing
  • coughing
  • speaking
  • singing

People with well-functioning immune systems may not experience TB symptoms, even though they are infected with the bacteria. This is known as latent or inactive TB infection. According to , about one-quarter of the world’s population has latent TB.

Latent TB isn’t contagious, but it can become an active disease over time. Active TB disease can make you and others sick.

Skin test

Your doctor can use a purified protein derivative (PPD) skin test to determine if you’re infected with the TB bacteria.

For this test, your doctor will inject 0.1 milliliter of PPD (a small amount of protein) under the top layer of your skin. Between two and three days later, you must return to your doctor’s office to have the results read. If there is a welt on your skin over 5 millimeters (mm) in size where the PPD was injected, you may be TB-positive. This test will tell you whether you have a TB infection; it doesn’t tell you whether you have active TB disease.

Reactions between 5 to 15 mm in size can be considered positive depending on risk factors, health, and medical history. All reactions over 15 mm are considered positive regardless of risk factors.

However, the test isn’t perfect. Some people don’t respond to the test even if they have TB, and others respond to the test and don’t have TB. People who’ve recently received the TB vaccine may test positive but not have TB infection.

Blood test

You doctor can use a blood test to follow up on TB skin results. The blood test may also be preferred over the skin test with certain health conditions or for specific groups of people. The two TB blood tests currently approved in the United States are Quantiferon and T-Spot. Blood tests results are reported as positive, negative, or indeterminate. Like the skin test, the blood test can’t indicate whether or not you have active TB disease.

Chest X-ray

If your skin test or blood test is positive, you will likely be sent for a chest X-ray, which looks for certain small spots in your lungs. These spots are a sign of TB infection and indicate that your body is trying to isolate the TB bacteria. If your chest X-ray is negative, you likely have latent TB. It is also possible your test results were incorrect and other testing may be necessary.

If the test indicates you have active TB disease, you will begin treatment for active TB. Otherwise, you will likely need to be treated for latent TB to prevent the bacteria from reactivating and making you and others sick in the future.

Other tests

Your doctor may also order tests on your sputum or mucus, extracted from deep inside your lungs, to check for TB bacteria. If your sputum tests positive, this means you can infect others with the TB bacteria and should wear a special mask until after you’ve started treatment and your sputum tests negative for TB.

Other tests such as a CT scan of the chest, bronchoscopy, or lung biopsies may be required if other test results remain unclear.

Many bacterial infections are treated with antibiotics for a week or two, but TB is different. People diagnosed with active TB disease generally have to take a combination of medications for six to nine months. The full treatment course must be completed. Otherwise, it’s highly likely a TB infection could come back. If TB does recur, it may be resistant to previous medications and be much more difficult to treat.

Your doctor may prescribe multiple medications because some TB strains are resistant to certain drug types. The most common combinations of medications for active TB disease include:

  • isoniazid
  • ethambutol (Myambutol)
  • pyrazinamide
  • rifampin (Rifadin, Rimactane)
  • rifapentine (Priftin)

These particular medications can affect your liver, so people taking TB medications should be aware of liver-injury symptoms, such as:

  • appetite loss
  • dark urine
  • fever lasting longer than three days
  • unexplained nausea or vomiting
  • jaundice, or yellowing of the skin
  • abdominal pain

Notify your doctor immediately if you experience any of these symptoms. You should also have your liver function checked with frequent blood tests while taking these medications.

Treatment for tuberculosis can be successful, given the person takes all the medication as directed and has access to proper medical care.

If the infected person has other diseases, it can be harder to treat active TB. For example, HIV affects the immune system and weakens the body’s ability to fight off TB and other infections.

Other infections, diseases, and health conditions can complicate a TB infection, as can insufficient access to medical care. Generally, early diagnosis and treatment, including a full course of antibiotics, offer the best chance for curing TB.

Most people in high-risk regions around the world receive TB vaccinations as children. The vaccine is called Bacillus Calmette-Guerin, or BCG, and protects against only some TB strains. The vaccine isn’t commonly given in the United States.

Having the TB bacteria doesn’t necessarily mean you’ll have symptoms of active TB. If you do have the infection and don’t show symptoms, you likely have latent TB. Your doctor may recommend a shorter course of antibiotics to keep it from developing into active TB disease. Common medications for latent TB include isoniazid, rifampin, and rifapentine, which may need to be taken for three to nine months, depending on the medications and combinations used.

People who’ve been diagnosed with active TB should avoid crowds until they are no longer contagious. According to , people with active TB can infect 10 to 15 people through close contact per year if they don’t take precautions.

People who are infected with active TB should also wear a surgical mask, known as a respirator, to keep TB particles from spreading through the air.

It’s best that a person with active TB avoid contact with others and continuing wearing a mask until instructed otherwise by their doctor.