Seven US counties under plague warning as experts say prairie dogs are good indicator of outbreak...

Health authorities in Colorado are warning residents in seven counties to be on guard against the re-emergence of plague in the region.

Though not nearly as fatal as it was in medieval times – when it killed an estimated 50 million people – the disease can still be deadly in the modern era. This year's bout of plague has already killed a 10-year-old girl in the state.

The plague returns to Colorado every summer, generally carried by fleas before being passed along to small animals and in some generally rare cases, humans. This year, San Miguel County, El Paso County, Boulder County, Huerfano County, Adams County and La Plate County have all reported cases of the plague.