A California woman has died and another resident is sick after coming down with Creutzfeldt-Jakob disease, a rare and fatal condition that swiftly destroys the brain.

The Marin County Department of Public Health is investigating the two cases, which are thought to be unrelated.

"We have no evidence that suggests a causal linkage between the suspect cases nor is there any evidence to suggest a risk in [the] food supply," the department said in a statement.

Creutzfeldt-Jakob disease is very rare, affecting roughly one in a million people each year, according to the U.S. Centers for Disease Control and Prevention. It is thought to be caused by an abnormal protein called prion, which erodes tiny holes in the brain, causing dementia and death in an average five months.

Most Creutzfeldt-Jakob cases are sporadic, meaning their cause is unknown, but a small fraction of cases are inherited. These cases are not contagious. A few very rare cases are linked to mad cow disease, which can be acquired through contaminated meat products, organ transplants and blood transfusions.

Laboratory tests have ruled out mad cow-related disease, called variant Creutzfeldt-Jakob, in one of the Marin County cases. The other case is still under investigation but public health officials suspect it, too, is unrelated to mad cow disease.

"While our investigation of both of these reported cases continues, we want to emphasize that we have no evidence of any environmental or public health risk in Marin County," Dr. Craig Lindquist, Marin County interim public health officer, said in a statement.

The patients' names and ages have not been released.

There have only been three variant Creutzfeldt-Jakob disease cases reported in the U.S., according to the CDC. In all three cases, the patient acquired the disease outside the country.