The following information is quoted directly from a USCIS Adjudicator's Field Manual:
"Tuberculosis, active. Only a Class A tuberculosis (TB) diagnosis renders an applicant inadmissible to the U.S. Under current CDC guidelines, Class A TB means tuberculosis that is clinically active and communicable."
LINK--> http://www.uscis.gov/sites/default/files/ilink/docView/AFM/HTML/AFM/0-0-0-1/0-0-0-17138/0-0-0-17143.html
For more information: http://www.uscis.gov/archive/archive-news/questions-and-answers-2009-update-tuberculosis-screening-required-adjustment-status-0