Screening for tuberculosis is necessary because tuberculosis is contagious, with smear-positive patients being the most contagious and bacillus-negative patients also being somewhat less contagious. Close contacts of tuberculosis need to have their lungs examined by plain film, so that they can be screened for tuberculosis at an early stage. In dense places like schools, if a TB-positive patient is found, the surrounding students must be screened. The most widely used screening tool in China is the tuberculin test, which can screen for TB infection, and if necessary, chest radiographs, which can detect some patients early, so TB screening is very necessary.