Participants whose PPD is negative have a second tuberculin test 10 to 21 days later and return 3 days after the test to determine if it is still negative or if it is positive. (Some people who are negative after the first test may test positive after the second procedure.) Those whose test is still negative end their participation in the study at that time. Participants whose second PPD is positive have a chest x-ray as described above, and those with a negative chest x-ray return in 3 weeks to donate one last blood sample.