Taylor Gaes had been infected with the plague, likely by fleas that put the bacteria directly into his blood stream, making it difficult to spot in time to stop its spread.
The 16-year-old 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 near Fort Collins hadn't been sickened by the fleas that could have infected him.
Some 200 people visited or attended the recent ash-scattering ceremony for the popular basketball and football player at the property. The incubation period for them expired Monday, said Katie O'Donnell, a spokeswoman for the Larimer County Department of Health and Environment.
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 Centers for Disease Control.
The last time someone died of the plague in the United States was in 2013 in New Mexico, a case of the less-common septicemic plague, the same form Gaes is believed to have had. Colorado's last plague death was in 2004 and it was also a case of septicemic plague, said Jennifer House, Colorado's public health veterinarian.
There are three main forms of the plague and all are caused by the same bacteria, Yersinia pestis, which is typically spread by fleas that have fed on infected rodents.
In the most common form, bubonic plague, the infection spreads through the body's tissue into the lymphatic system, producing tell-tale swelling of the lymph nodes. 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.
Most U.S. cases are in the West where rodents, including prairie dogs, carry the disease. Cases are most common in the Four Corners region where Colorado, Arizona, New Mexico and Utah meet but cases have also been reported as far north as Canada, said Paul Ettestad, the state public health veterinarian for New Mexico. The last person to die of the plague there was a person with the septicemic variety in 2013.
In Larimer County, where Gaes lived, no resident has contracted plague since 1999. So, without any swelling, it's not something that would typically be on people's minds, O'Donnell said.
"It's very hard for anyone to look at someone and say that's a disease we only have every 10 or 15 years," she said.
County health officials are working with the CDC to test wildlife on the family's property to see if the animals have the plague. If tests are negative, O'Donnell said it's likely that investigators will never know for sure the source of Gaes' infection.
AP medical writer Lindsey Tanner contributed to this report.