spread through the lymph system and the bloodstream to other sites in the lungs as well as the brain, bone, and kidneys. The most common site of infection—and active tuberculosis—is the upper part of the lungs.
As they spread, the tuberculosis bacteria will usually provoke a further, more powerful infection-fighting response that physically contains the bacteria in hard, immune-cell clusters called granulomas. (Tuberculous granulomas are called tubercles.) This process normally stops further multiplication and spread of the bacteria. Some bacteria may, however, remain alive within a granuloma for years, even decades. This condition is described as latent infection with M. tuberculosis. As described below, the surviving bacteria have the potential to cause active tuberculosis at a later time. A successful immune response to tuberculosis bacteria generally takes between 2 and 10 weeks to develop.
The great majority of those with latent tuberculosis infection never develop active disease, have no symptoms, and do not infect others. No national data on tuberculosis infection are available. Estimates prepared for the Occupational Safety and Health Administration (OSHA) put the prevalence of latent tuberculosis infection in the United States in 1994 at about 6 to 7 percent of the population over age 18 years (62 FR 201, October 17, 1997). As discussed in Chapter 5 and Appendix C, rates of infection and disease vary considerably among racial, ethnic and other population subgroups.
It is often stated that an estimated 10 percent of those infected with M. tuberculosis will develop active tuberculosis if their latent infection is not treated (see, e.g., CDC [2000a]). This number has, however, been questioned (see Chapter 7 and Appendix C), and it likely overstates the risk for most workers, in part because the rate of progression from untreated infection to active disease varies by age. Studies conducted in the 1960s and 1970s indicate peaks in progression for young children (approximately ages 1 to 4) and for those in adolescence and early adulthood (approximately ages 13 to 24) (Comstock, 2000) .7 Rates for working-age adults are lower.
Studies suggest that the highest risk of progression to active disease is during the first 1 to 2 years following infection with M. tuberculosis. As discussed further below, treatment of latent tuberculosis infection substantially reduces the risk of progression to active disease.