Three members of a Calvert County family died in the past week following respiratory illnesses, two from complications from the flu, according to the chief medical officer at MedStar Washington Hospital Center, where a fourth family member is being treated.

The family was from the Lusby area, according to hospital and county officials, who on Tuesday issued a press release about the investigation into the deaths.

The first case involved an 81-year-old woman who grew sick at home around Feb. 23. Her son and two daughters took care of her there, and then developed similar upper respiratory symptoms five days later, on Feb. 28, health officials said.

The mother became sick with traditional flu symptoms and also had other underlying medical conditions, according to Janis Orlowski, chief medical officer at the hospital center, located in the District. The mother died at home March 1.

Her 58-year-old son and a 56-year-old daughter initially were hospitalized at Calvert Memorial Hospital. Both died Monday — the daughter at Calvert and the son at the hospital center, where he had been transferred.

The other daughter, 51, was transferred from Calvert to the hospital center Monday and is “doing better today,” Orlowski said.

That woman arrived with the same flu-like symptoms as her siblings, including fever, aches, cough and shortness of breath, Orlowski said.

Tests confirmed the siblings who died had a strain of flu virus known as influenza A, and each also acquired a serious staph infection, according to Orlowski. She said it was unlikely the infection was acquired in the hospitals because the siblings arrived coughing blood. “It’s likely they came to the hospital with the infection, which is what caused the cough and fever,” Orlowski said.

The mother was Lou Ruth Blake, according to a funeral home notice that was confirmed by a county official who did not want to be identified because the investigation is not closed.

Tests are being conducted to determine whether the second daughter has the same influenza virus and infection. Officials at the Centers for Disease Control and Prevention will determine what strain of the influenza A virus infected the family, Orlowski said.

This year’s flu season has gotten off to a late start, but Orlowski said there has been an increase in the number of patients with flu-like symptoms in recent weeks. Hospital center officials are looking into at least one other recent death — unrelated to the Calvert cluster— to determine whether it was related to flu.

The severity of illness and death due to flu varies widely from season to season. The very young, adults older than 65 and individuals with certain medical conditions are at higher risk for developing flu-related complications.

The 2011-2012 flu vaccine will protect against the three viruses that research indicates will be most common during the season: two strains of the influenza A virus, H1N1 and H3N2, and an influenza B virus.

Health officials said the department is urging people who have not yet received a flu vaccine to get one. Local officials are also asking residents to take standard precautions to prevent the spread of illness, including hand washing and limiting contact with sick individuals. Anyone with flu-like symptoms should check with a health-care provider.

Maryland’s Office of Chief Medical Examiner also is conducting autopsies as part of the investigation. Medical Examiner David Fowler said Calvert health officials contacted his office Monday seeking assistance. One of the bodies was being transported to Baltimore on Tuesday for an autopsy. At least one of the bodies had already been embalmed and buried, local officials told him.

The medical examiner’s office will examine the respiratory organs in detail, Fowler said, and conduct blood cultures for bacteria and viruses.

Staff writer Bonnie Smith and research director Madonna Lebling contributed to this report.