What is tuberculosis?
Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis bacteria. TB primarily targets the lungs but may affect any area of the body such as the urinary tract, central nervous system, bones and/or joints, other organs, abdominal area, or lymph nodes. A tuberculosis infection that has spread through the blood to various organs of the body is termed miliary tuberculosis. With respiratory infections, TB is spread through the air from person to person through droplets of respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, or breathing. Most of those who become infected with M. tuberculosis manage to confine the mycobacteria to a few cells in their lungs, where they stay alive but in an inactive form. This latent TB infection does not make the person sick or infectious and, in most cases, it does not progress to active tuberculosis.
However, some people - especially those with compromised immune systems - may progress directly from initial TB infection to active tuberculosis. People who have HIV are much more likely to become sick if they contract TB. In another 5-10% of those with latent TB infection that has not been treated and who are not co-infected with HIV, the mycobacteria will later be reactivated and begin to multiply, leading to active progressive tuberculosis disease.
TB has been a leading cause of death for thousands of years. In the days before the discovery of antibiotics, it was called consumption, and those who contracted it were put into long-term hospitals called sanatoriums for the rest of their lives. Although its frequency has decreased dramatically in the United States, the World Health Organization (WHO) estimates that one-third of the world's population is currently infected with M. tuberculosis. Worldwide, TB is still a leading cause of death due to an infection, especially in those who are HIV-positive, killing about 1.7 million people a year.
In the US, there were over 11,000 reported cases of TB in 2009. This represents a rate of about 3.8 cases per 100,000 persons, the lowest recorded since reporting began in 1953. There was resurgence in new cases in the early 1990s, but the rate of TB infections has been steadily declining since then. Nevertheless, certain populations within the US are at higher risk of infection. These include people born outside the US as well as certain racial and ethnic populations. People infected with both HIV and M. tuberculosis have been shown to be at a significantly increased risk of developing active TB and of dying from it. While the numbers of new cases of active TB have again declined in the U.S. due to constant vigilance by the medical community, TB remains a significant national and global public health problem.
Often, new cases of TB are resistant to the antibiotics typically prescribed to treat the disease (multidrug- resistant, MDR), making the infections more difficult to cure. Extensively drug-resistant tuberculosis (XDR-TB) is an emerging concern. It is even more resistant and difficult to treat than MDR-TB. XDR-TB has been defined by WHO and the Centers for Disease Control and Prevention as M. tuberculosis that is resistant to the drugs isoniazid and rifampin, to the drug class fluoroquinolone, and to at least one of three injectable "second-line" drugs (amikacin, kanamycin, or capreomycin). Cases of XDR-TB are being closely monitored by the world medical community and measures are being taken in hopes of limiting its spread.