Taylor Gaes of Livermore died June 8, but the cause was only made public late Friday when health officials, at the urging of the boy's parents, put out a warning to make sure others who may have visited his family's rural home northwest of Fort Collins hadn't been sickened by fleas that could have infected him.
Those visitors are now considered safe because the incubation period for the disease passed on Monday.
Gaes likely was infected with a rare form of the disease, septicemic plague, by fleas that put the bacteria directly into his blood stream, making it difficult to spot in time to stop its spread.
Larimer County and CDC officials said Tuesday they likely won't ever know how Taylor was infected but were canvassing the area to try to gauge the plague's presence. They warned residents to take common precautions to avoid flea bites and to stay away from dead animals and rodents.
Taylor's father, Shannon Gaes, said his family has lived in the Livermore area for 11 years and didn't know that plague existed there.
"We don't know where he got it," Shannon Gaes told a news conference. "So we just want people to be aware. A lot of people go hunting, fishing, camping and stuff like that up there."
Cases of the plague are rare and deaths are even rarer. Nationally, an average of seven human plague cases is reported each year, with an average mortality rate of 11 per cent, according to the CDC.
The most common form, bubonic plague, affects the lymphatic system, producing tell-tale swelling of the lymph nodes. All types can be treated and cured when antibiotics are given soon after infection, but all of them are deadly when treatment is delayed.