In our modern society, diseases can spread quickly around the globe. To prevent outbreaks in the United States like the current Ebola epidemic in Africa, the Immigration and Nationality Act (INA) makes inadmissible any person who has a communicable disease of public health significance or has not had required vaccinations.
Whether a person falls into one of these categories is determined by the language of the INA and corresponding regulations.
At this time seven diseases are considered to be "of public health significance": chancroid, gonorrhea, granuloma inguinale, infectious leprosy, lymphogranuloma venereum, active tuberculosis, and infectious syphilis. However, a number of other diseases are incorporated by reference, including cholera, diphtheria, infectious tuberculosis, plague, smallpox, yellow fever, viral hemorrhagic fevers, SARS, and certain types of influenza. In addition, communicable diseases that may pose a "public health emergency of international concern," as determined by the Center for Disease Control, may also create inadmissibility.
Required vaccinations include mumps, measles, rubella, polio, tetanus, diphtheria, pertussis, influenza type B, and hepatitis B.
It is possible to obtain a waiver of inadmissibility if you fall into one of the categories described above. It is important to consult with an immigration attorney who can assess your specific situation to make that determination.
On August 13, 2014, the Congressional Research Service released a report titled Immigration Policies and Issues on Health-Related Grounds for Exclusion. The report provides a brief history and description of health-related grounds for exclusion, as well as a discussion of required medical exams and waivers available for inadmissibility. You can read the full report here: