Strongly positive PPD chest x-ray is normal may be caused by Mycobacterium tuberculosis infection, BCG vaccination, previous infection, etc. 1. Mycobacterium tuberculosis infection: strong positive PPD indicates the presence of Mycobacterium tuberculosis infection in the body. However, the tuberculosis bacteria have not yet affected the function of the lungs and the chest X-ray is normal. 2. BCG vaccination: after the patient has been vaccinated with BCG vaccine, the vaccine will stimulate the immune system, and then a strong positive chest radiograph of PPD can also appear normal. 3. Previous infection: when the patient has been infected with tuberculosis bacillus, after curing the disease, there will be corresponding substances in the body, and when doing the examination, it will also be found that the PPD strong positive chest X-ray is normal. People with normal PPD strong positive chest radiographs should go to the hospital regularly for corresponding examinations.