The United States has seen a decrease in the incidence of active tuberculous disease (TB) over the past several decades. However, TB continues to circulate in the US. While certain epidemiologic factors and/or comorbidities are associated with increased risk of TB, including being born outside the US, living with HIV, or having a history of incarceration or homelessness, not all new TB diagnoses are in individuals with these risk factors.

M. tuberculosis is usually transmitted only through air, and not by surface contact. It is carried in airborne particles that can be generated when persons who have pulmonary or laryngeal TB disease cough, sneeze or shout. The probability that a person who is exposed to M. tuberculosis will become infected depends on the concentration of infectious particles in the air and the duration of exposure to a person with infectious TB disease.

After initial infection, the immune response usually limits additional multiplication of the tubercle bacilli. However, some bacilli remain dormant in the body for years. This condition is referred to as latent tuberculosis infection (LTBI). Persons with LTBI are asymptomatic and cannot spread TB, but they can reactivate and develop active TB disease. Reactivation occurs in 5-10% of infected persons over their lifetime with the highest risk in the first several years after infection. When reactivation occurs, the individual becomes symptomatic and is capable of transmitting infection to others. Individuals with impaired immunity (from disease or medications) have a higher rate of progressing to active tuberculosis.

In the United States, LTBI has been traditionally diagnosed using a Purified Protein Derivative (PPD)-based Tuberculin Skin Test (TST). More recently, blood tests called interferon gamma release assays (IGRA; e.g. T-spot, quantiferon) have emerged as another way to test for LTBI. Individuals who received a BGC vaccine (given in some countries outside the US) may have a false positive PPD TST however IGRAs can reliably be used in this population. TSTs and IGRAs become positive within 1-12 weeks after an exposure. These tests should NOT be used to rule out active tuberculosis, as there are too many false positives and false negatives for the test to be reliable. The CDC no longer recommends routine annual TB testing of healthcare personnel; such testing should only be done if there is ongoing transmission at a healthcare facility or if there are particular groups at high risk.