Tuberculosis is an infection with the bacterium Mycobacterium tuberculosis, which most commonly affects the lungs (pulmonary TB) but can also affect the central nervous system (meningitis), lymphatic system, circulatory system (miliary TB), genitourinary system, bones and joints.
Tuberculosis is the most common major infectious disease today, infecting two billion people or one-third of the world's population, with nine million new cases of active disease annually, resulting in two million deaths, mostly in developing countries.
Most of those infected (90 percent) have asymptomatic latent TB infection (LTBI). There is a 10 percent lifetime chance that LTBI will progress to active TB disease which, if left untreated, will kill more than 50 percent of its victims. TB is one of the top three infectious killing diseases in the world: HIV/AIDS kills 3 million people each year, TB kills 2 million, and malaria kills 1 million.
The neglect of TB control programs, HIV/AIDS, and immigration has caused a resurgence of tuberculosis. Multiple drug resistant strains of TB (MDR-TB) is increasing. The World Health Organization declared TB a global health emergency in 1993.
TB is spread through aerosol droplets which are expelled when persons with active TB disease cough, sneeze, speak, or spit. Close contacts (people with prolonged, frequent, or intense contact) are at highest risk of becoming infected (typically 22 percent infection rate but everything is possible, even up to 100%). A person with untreated, active tuberculosis can infect an estimated 20 other people per year. Others at risk include immigrants or visitors from areas where TB is common, residents and employees of high-risk congregate settings, health care workers who serve high-risk clients, medically underserved, low-income populations, high-risk racial or ethnic minority populations, children exposed to adults in high-risk categories, and people who inject illicit drugs.
Transmission can only occur from people with active TB disease (not latent TB infection).
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