A seven-year-old girl in Colorado is recovering from bubonic plague, the illness that spread across Europe in the 14th century, killing tens of millions of people.

Sierra Jane Downing was camping with her family in late August when she came across a dead squirrel.

Her mother, Darcy, told her to stay away from the animal but she said that her daughter just wanted to bury it.

Days later, Sierra became very ill, prompting her frightened father to rush her to hospital.

The doctors who treated her admitted that the unusual symptoms were initially puzzling, but when they diagnosed bubonic plague they immediately started the treatment as time can make all the difference.

"I did tell the parents that I'm always hopeful that everything is going to turn out for the best but I was trying to prepare them somewhat that she was critically ill and this is a life-threatening condition that she had and we were doing absolutely everything we could to save her life," said Dr. Snow.

Doctors expect Sierra to make a full recovery and said she may be discharged within the week.