A 16-year-old boy in Colorado who appeared to have the common flu has died from a rare case of the plague, officials said.

Taylor Gaes’ illness didn’t present with the telltale sign of the infection – swollen lymph nodes – which would have alerted officials to the illness sooner, said Katie O’Donnell, a Larimer County Health Department spokeswoman.

Instead, he suffered from a fever and muscle aches, which at first made his sickness look like the flu.

The plague is “very rare, which makes it hard to diagnose,” O’Donnell said Saturday. Taylor died June 8 but officials revealed his illness Friday.

Officials are unsure which type of plague Taylor contracted, though they suspect bubonic plague because it is the most common and the easiest to transmit through a bug bite. Taylor likely encountered a flea from a sick rodent that wandered onto his family’s property from a neighboring rural area.

The U.S. counts an average of seven human cases of plague each year and fatalities are rare. The CDC and local health departments monitor rodent populations where plague occurs for spikes in animal deaths from the disease.