ATLANTA (AP) -- A 90-year-old nursing home patient died from the stomach flu last year, marking the first time U.S. health officials confirmed that the highly contagious bug is sometimes fatal.

The North Carolina woman so far is the only person for whom lab tests confirmed norovirus as the killer, but health officials believe the virus killed at least 18 others and caused thousands of illnesses late last year.

Noroviruses are a group of viruses that cause the stomach flu, or gastroenteritis. They also are sometimes called Norwalk-like viruses.

Health officials don't systematically count and diagnose norovirus cases. But last winter seemed to be particularly nasty, with more than 1,300 outbreaks reported in 24 states, according to the U.S. Centers for Disease Control and Prevention.

Two new strains of norovirus were identified in the outbreaks, and may have driven the heavy caseloads, according to an article this week in a CDC publication, Morbidity and Mortality Weekly Report.

It isn't known which strains caused the deaths, said Jackie Tate, a CDC investigator who co-authored the article.

In recent years, cruise ships have become famous norovirus carriers, with several large outbreaks grabbing headlines. But nursing homes and other long-term care facilities were the scene of many of last winter's outbreaks, she said.

Proof that the virus killed the North Carolina patient is significant but not surprising, said Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University.

"It is likely norovirus has been responsible for at least some fatal infections in the past," Schaffner said.

The virus is particularly dangerous for the very old, the frail and the very young, he added.

Symptoms include nausea, vomiting, diarrhea and stomach cramping. Some people also suffer a low-grade fever, chills, headache, muscle aches and tiredness. The illness often begins suddenly, but in most people symptoms last only one or two days.

There's no good treatment for norovirus. Doctors usually advise drinking fluids and try to treat the symptoms, Tate said.

The virus is spread through food, personal contact and touching contaminated surfaces. It's not clear how the virus spread in many of the nursing home settings, Tate said.

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