Travel medicine physicians say that figuring out which vaccinations are needed for travel is not easy, and doing so depends on factors such as health, destination, and previous immunizations. It is ideal to check which shots are needed months ahead of time, although some vaccinations can be given at the last minute, if necessary. The Centers for Disease Control and Prevention's Web site has a list of immunizations by destination and a section that lists diseases by their occurrence in a region. The World Health Organization's Web site also has a list of immunization requirements, and the International Society of Travel Medicine offers a listing of travel medicine clinics in over 40 nations. The most commonly recommended vaccines include those for yellow fever, typhoid fever, hepatitis A, and influenza. Other needed vaccines may include polio, rabies, and tetanus.