Measles outbreaks continue to pop up across the United States, with the number of confirmed cases of the once-eradicated disease now exceeding 700 in 22 states — but so far, only one person in Colorado has become ill with the highly contagious virus. In January, a Denver resident was placed in isolation and treated for the respiratory illness after being diagnosed following a trip abroad, leading local health officials to issue a public warning about possible exposure to others. It’s the only confirmed case of measles in Colorado this year, but health experts said the state’s notoriously low vaccination rate makes communities here vulnerable to a possible outbreak. “We’ve seen it happen over and over in communities where the vaccination rate is low,” said Dr.