There have been 11 cases of human plague in the U.S. since April 1, according to the CDC. Here are five things to know about plague in the U.S.
1. The CDC is unsure why there have been so many human plague cases this year. "It is unclear why the number of cases in 2015 is higher than usual," a CDC statement reads. From 2001 to 2012, the number of plague cases in the U.S. has ranged from one to 17, with a median of three per year.
2. In the antibiotic era, the mortality rate for plague is roughly 16 percent. Of the 11 cases this year, three patients (ages 16, 52 and 79) have died.
3. Plague was introduced in the U.S. in 1900, thanks to rats that were on-board steamships from affected areas, according to the CDC. The last outbreak of plague in the U.S. happened in Los Angeles from 1924 through 1925.
4. Today, most human plague cases in the U.S. happen in two regions: northern New Mexico, northern Arizona and southern Colorado; and California, southern Oregon and far-western Nevada. This year, cases have happened in six states: Arizona, California, Colorado, Georgia, New Mexico and Oregon.
5. The CDC says healthcare providers should "consider the diagnosis of plague" in patients that have symptoms compatible with the plague, who live in or have travelled to the western U.S. and who have had recent proximity to rodents, rodent habitats or sick domestic animals. If a clinician suspects plague, they should collect samples for diagnostic testing, implement isolation and respiratory droplet precautions, start immediate antibiotic testing and notify public health officials.