New testing at George Bush High School has found 10 new cases of tuberculosis.
Fort Bend County Health and Human Services is investigating of contacts to active TB at the school.
Ten previous TB cases were found at the school in June after students and teachers alike were tested.
The health department said in statement that it's not possible know that all 20 of the infections are related to the active cases at George Bush High School or from other exposures.
"However, all can be evaluated and offered preventive medication," the agency states. "As a reminder, a positive test does not mean that the person is ill with active TB disease; it simply means that they have been exposed to the bacteria and are infected."
TB is caused by bacteria that causes disease in the lungs -- but it also can affect other organs of the body.
TB can be spread from person to person, but usually takes close and prolonged close contact with a person who has active TB.
It does not spread as easily as a cold, or flu or measles -- but it can spread if coughed or sneezed.
TB symptoms of tuberculosis include persistent and productive cough lasting more than two weeks, fevers, night sweats, weight loss or coughing up blood.
Students and faculty members have been told to come to campus and get a TB test before the school year begins.