Many of these diseases still thrive in other parts of the world. Travelers can and do bring these viruses back to the U.S. Without the protection of vaccines, these diseases could easily spread here again.
A major measles outbreak started in West Texas in January 2025 and spread around the U.S. Children died from this preventable infection. The infection primarily occurred in unvaccinated or undervaccinated children and adults, and in those with weakened immune systems.
Due to undervaccination, the U.S. is also experiencing increased cases of other vaccine-preventable illnesses, such as Whooping Cough (Pertussis) and Polio.
To best protect the population against these serious illnesses, a large percentage (greater than 90% to 95%) of the people in an area need to be immune, either from vaccination or prior infection. Otherwise, one of these highly infectious diseases, once introduced into a population, can spread widely among those who are not protected and others who are at higher risk due to weaker immune systems (such as the elderly, those with cancer or transplanted organs, or those taking immune suppressing medicines).