A travel alert has been issued warning Americans to take precautions against polio, which is spreading in Europe and elsewhere across the globe. The U.S. Centers for Disease Control issued a level 2 alert, cautioning travelers to "practice enhanced precautions" before visiting 32 countries. The agency is advising people to make sure they're up to date on their polio vaccines, adding that people who plan to travel to the listed countries are eligible for a single-dose booster of the vaccine.