New Mexico health officials did not immediately say Thursday how the brother and sister contracted the infectious disease, but they are conducting an investigation at the family's residence to determine if there is any risk to other people.
Plague is generally transmitted to humans through the bites of infected fleas, but also can be transmitted by direct contact with infected animals, including rodents, rabbits and pets.
Symptoms of the bubonic form of the plague in humans include fever, chills, headaches, vomiting, diarrhea and swollen lymph nodes in the groin, armpit or neck areas. Pneumonic plague, which is an infection of the lungs, can include severe cough, difficulty breathing and bloody sputum.
The Health Department, citing privacy concerns, would not release the name of the siblings or give a location for their home, other than saying it was in Santa Fe County. Spokeswoman Deborah Busemeyer said the boy died in the last couple of days but she declined to be more specific.
Fleas collected from the area are being sent to the Centers for Disease Control and Prevention for testing. Health workers also canvassed the neighborhood to tell other residents that plague had been confirmed in the area.
The CDC says an average of 10 to 15 persons contract the plague each year in the United States. Modern antibiotics are an effective treatment.