Immunization is one of the most important medical precautions one can take when traveling.
Diseases and infections such as diphtheria, measles, and Haemophilus influenzae type b are rare in the United States because of routine immunization. However, they are more common in developing countries where routine immunization programs may not exist. In the past, travelers were required to have a certificate proving they had received certain immunizations. Today, immunization is less formal, even though travelers are no less at risk. Immunization requirements have been replaced by recommendations, making most travelers less likely to seek advice from their healthcare professional.