Colorado Already Has 2 Plague Deaths This Summer Officials believe the victims were infected by fleas By Newser Editors and Wire Services Posted Aug 6, 2015 4:19 PM CDT 1 comment Comments Doctors believe both people killed by plague this year in Colorado were infected by fleas. (Shutterstock) (Newser) – A Colorado resident died of the plague Tuesday, likely after being infected by fleas from a dead animal. The unidentified adult becomes the second person to die from the infamous disease this year. A 16-year-old star baseball player died of the plague in June. Both victims likely had a rare septicemic form of the plague, which is hard to diagnose because it doesn't include the typical swelling of lymph nodes seen in most plague cases. Cases of the plague are rare in the United States, and deaths are even rarer. On average, seven human plague cases are reported in the United States every year, but Colorado had eight plague cases of its own last year. Officials are attributing that to a boom in the animal population. In septicemic plague, the bacteria enters the blood stream and creates flu-like symptoms. The most common type of plague is the well-known bubonic. All types of plague can be treated but are deadly when that treatment is delayed.