And every person entering the country "LEGALLY" has to show proof of vaccinations. The only people entering this country with a chance of spreading these diseases are ILLEGAL aliens.
There are no vaccination requirements for visitors to the United States. For information on US-based infectious disease outbreaks currently being reported by CDC, please see the CDC Current Outbreak List.
http://wwwnc.cdc.gov/travel/destinations/traveler/none/united-states
Really?
It is not unusual for countries to require certain vaccinations for travelers to enter, but I do not know of any requirements for travelers to the US. In fact, the CDC says, "There are no vaccination requirements for visitors to the United States."
Also, measles outbreaks have occurred when unvaccinated Americans have traveled to places like the Phillipines, where measles is common.
Sometimes, the unvaccinated can have a mild case of the disease, but they are still contagious and can spread it to those who are too young to be vaccinated or who are immunocompromised.