No one doubts that chikungunya is making its way, person by person, to the United States; more than 12 million Americans visit the Caribbean each year, and so far dozens bearing chikungunya have returned to Indiana, Florida, and other states. Given air travel patterns and mosquito concentrations, a study in June put the entire East Coast, from New York City southward, at risk. If an outbreak doesn’t arise from the blood of an infected traveler, the virus could simply travel up through Central America and Mexico.