Tuberculosis rates in prisons and jails typically are five times higher than in the general population. Inmates and even correctional health care workers are more likely to get TB than people in the general population because correctional facilities house a higher percentage of former substance abusers. HIV infections also occur in greater percentages in prisons and can hasten the spread of TB. Preventing the spread of TB in a correctional facility involves isolation of infected persons and aggressive medical treatment. A Missouri program involves a two-step skin test when inmates enter a correctional facility. Inmates testing positive for the disease are placed on a regimen of two or three drugs for 6 to 12 months, and are isolated in a negative air pressure system which draws air from the general prison facility but does not put air from the isolation area back into the general area. Health care staff members continually monitor patients' progress to determine when the danger of contagion is past and it is safe to end the isolation.