A person from southern Colorado has died of the plague, the second person to die of the disease in the state this year.

The unidentified adult from Pueblo County likely died of the relatively rare septicemic form of the disease, said Christine Nevin-Woods, the medical officer for the Pueblo City-County Health Department. Officials believe the person likely was infected by fleas from a dead rodent or another animal.

In June, a 16-year-old star baseball player from northern Colorado also died of septicemic plague that wasn't detected until after he passed away. That's a relatively rare form of the plague that is difficult to diagnose because it isn't accompanied by the tell-tale swelling of lymph nodes seen in most plague cases.

Cases of the plague are rare in the United States and most common in the West. Deaths are even rarer. Nationally, an average of seven human plague cases is reported each year, although Colorado had eight on its own last year. The state has seen in a rise in the number of cases spread by small animals this year, including rabbit fever, after a wet spring created unusually lush vegetation that supported a boom in the animal population.

The most common form of the plague is bubonic, in which the infection spreads through the body's tissue into the lymphatic system, producing swelling. In septicemic plague, the bacteria directly enters the blood stream and, without the swelling, its symptoms of fever, chills and abdominal pain, can look like the flu.

The third and least common type is pneumatic plague, which involves pneumonia and is capable of spreading among people through coughing.

While all types can be treated and cured when antibiotics are given soon after infection, all of them are deadly when treatment is delayed.

Until this year, the last time someone died of the plague in the United States was in 2013 in New Mexico. Colorado's last previous plague death was in 2004. Both the New Mexico and that Colorado case were septicemic plague cases.