A woman in her early 40s was hospitalized for two nights last week with a confirmed case of H1N1 influenza, or swine flu, Martha’s Vineyard Hospital chief executive officer Timothy Walsh confirmed late yesterday.

This is the first confirmed incidence of the virus in Dukes County. The case means that every Massachusetts county has now had at least one confirmed case, the Massachusetts Department of Public Health reported yesterday.

It is unusual for an individual with H1N1 to be hospitalized; that is usually linked to another health complication, said Edgartown health agent Matthew Poole.

The hospital could provide only scant details and did not put out a public statement about the case, despite the fact that the rare and sometimes deadly flu virus has attracted widespread national attention since April when a public health emergency was declared.

The hospital could not confirm why the woman, whom Mr. Poole said was an Oak Bluffs resident, had been admitted. But a nursing supervisor on duty last night said it was possible the patient was dehydrated or admitted for observation as a precaution.

The patient was discharged on Friday, Mr. Walsh believed.

“We get a lot of flu cases in the winter,” he said. “I knew [this first case of swine flu on the Island] wasn’t severe.”

Mr. Walsh said the hospital was prepared for the virus, and no unusual precautions were required.

Health agents in all towns were notified on Friday that an Oak Bluffs resident had tested positive for the virus, Mr. Poole said.

This was one of 86 additional H1N1 cases in Massachusetts reported yesterday, bringing the confirmed case count to 940. Eight of the newly confirmed cases were hospitalized. The total cumulative number of hospitalizations is now 65, according to the state health department’s update yesterday.

The new virus spreads from person to person in much the same way that regular seasonal influenza viruses spread. There have been isolated cases in previous years, but in March and April this year, cases increased dramatically in Mexico and the United States. American health officials declared a public health emergency in late April.

According to pandemicflu.gov: “The symptoms of H1N1 (swine) flu in people are similar to symptoms of regular human flu and include fever, cough, sore throat, body aches, headache, chills and fatigue. Some people have reported diarrhea and vomiting associated with H1N1 (swine) flu. In the past, severe illness (pneumonia and respiratory failure) and deaths have been reported with H1N1 (swine) flu infection in people. Like seasonal flu, H1N1 (swine) flu may cause a worsening of underlying chronic medical conditions.” The commonwealth’s department of public health notes: “Flu outbreaks evolve in unpredictable ways, it is impossible to know whether this outbreak will decrease, remain the same, or grow in coming weeks, and whether the illness will remain at its current severity which, on the whole, has been relatively mild.”

Mr. Poole said he expects that the Island can expect to see more confirmed cases as the seasonal populations grows.

He said that Island health agents were prepared and in regular communication. They had met with school officials and agreed on plans in case the outbreak affects Island schools. “We have had a sense of what might evolve,” Mr. Poole said. “We had thought through it . . . we have met and worked together regularly since 9/11,” when the federal government required more pandemic preparedness and planning.

The state department of public health notes that because there is no vaccine for H1N1 influenza, public health officials suggest people take simple steps such as frequent hand-washing with soap and warm water or alcohol-based hand sanitizer; coughing into a tissue or inner elbow and not into your hands; if you are sick stay home from work and if your child is sick keep them home from school; and stay informed about the latest developments on the H1N1 flu.

For more information on swine flu, see online mass.gov/dph.