The sudden outbreak of measles across the United States raises questions about how the disease arrived in the country after it was eradicated here in 2000.

A quick review of the U.S. State Department visa requirements for immigrant visas reveals that applicants must be vaccinated for measles prior to their approval. Measles vaccination is not required for tourist visas, according to the Centers for Disease Control and Prevention, nor is it required for any other disease.

That leaves three possibilities. One is that a tourist present legally in the U.S. brought measles to the country while visiting. This is the most plausible explanation, given that the outbreak began at Disneyland.

Another possibility is that an illegal immigrant brought measles to the country.