The Centers for Disease Control and Prevention has technical instructions for medical examination of prospective immigrants in their home countries before they are permitted to enter the U.S. They are screened for communicable and infectious diseases such as tuberculosis, malaria, hepatitis, polio, measles, mumps and HIV. They are also tested for syphilis, gonorrhea and other sexually transmitted diseases. The CDC also has medical screening guidelines for refugees. These screenings are usually performed 30 to 90 days after refugees arrive in the United States.