A report this week about a Seattle woman who died from a brain-eating amoeba after using a neti pot has users of the nasal irrigation device worried. But doctors say the infection is exceedingly rare, and note that careful use of the neti pot is very important.

The 69-year-old woman, who had a chronic sinus infection, used tap water filtered by a Brita water purifier, according to a report published in the International Journal of Infectious Diseases.

“Fortunately, these amoebic infections are quite rare, but we have documented that some have occurred due to use of tap water in neti pots," Dr. Jennifer Cope, a medical epidemiologist for the Centers for Disease Control and Prevention, told NBC News on Friday. "The message we want people to hear is that if you use the neti pot you should not use tap water directly."

Sterile water, which is free of bacteria and viruses, is recommended for use in neti pots. Tap water can be used only if it has been passed through a special filter or boiled for three to five minutes, then left to cool until lukewarm. Most bottled “spring” water is not considered sterile, so it should not be used unless it is boiled first.

The Seattle Times reported Thursday that the woman, who was not identified, was admitted to the city's Swedish Medical Center in January after suffering a seizure. An examination of her brain tissue showed that it was infected with Balamuthia mandrillaris, an amoeba found in soil and fresh water. It had been eating her brain for almost a year, the Times said.

“When I operated on this lady, a section of her brain about the size of a golf ball was bloody mush,” Dr. Charles Cobbs, a neurosurgeon at Swedish, told the Times. “There were these amoeba all over the place just eating brain cells. We didn’t have any clue what was going on, but when we got the actual tissue we could see it was the amoeba.”

Balamuthia can affect almost any organ in the body, but it often causes a rare and serious infection of the brain and spinal cord called granulomatous amebic encephalitis. A combination of drugs are often given to relieve symptoms, but there is no cure. The infection is fatal in about 90 percent of cases.

Early diagnosis and treatment may increase the chances for survival.

Arrows point to the amoebas in the Seattle woman's brain. Dr. Sean Thornton / Swedish Medical Center

Balamuthia infections are very rare. Since 1962, about 200 cases have been reported around the world, the CDC says. From 1974 to 2016, there were 109 cases in the U.S. The number of cases ranges between zero to nine a year.

"There may be cases that go undiagnosed because the disease mimics other brain infections, but we don’t suspect that the number is large," said Cope.

The low rate of infection may be due in part to the amoeba’s high fatality rate, which decreases the chance of it spreading from one person to another.

Little is known about how people contract the infection and how to prevent it. The most common sign is a rash on the face, trunk or limbs. Other early symptoms include headache, stiff neck, sensitivity to light, nausea and vomiting, and fever.

Once the infection spreads to the brain, seizures, speech difficulties, paralysis, and difficulty walking are typical.

How to use a neti pot

A neti pot is a container designed to rinse debris or mucus from the nasal cavity. People use it to treat symptoms of nasal allergies, sinus problems or colds.