When Lou Ruth Blake and two of her adult children died within days of one another from a complication of influenza, the loss was felt in every corner of the close-knit Calvert County community in which they lived. It was not simply a matter of being well liked or well known in Lusby. It was a matter of blood.

Across the street from Blake’s white Cape Cod lives a great niece. Next door to the great niece is a brother-in-law. And the next few houses in either direction are occupied by cousins of her late husband, Leroy Blake.

The Blakes’ roots run deep here. They have their own folder at the county historical society. They were among the earliest members of a nearby Methodist congregation that dates back to the end of the 19th century.

The Blakes married some of the other congregants. And the headstones in the cemetery next to Eastern United Methodist Church bear the names of those interconnected families, just as do the mailboxes that line the roads near the church.

Many members of the extended family stopped by March 1 — not long after Ruth Blake, 81, died — to be together and to pray.

A family member had asked her pastor, the Rev. Irving Beverly, to come to pray as well. Inside, he was surprised to see two sheriff’s deputies. At that point, the cause of Blake’s death was unknown.

Two of Ruth Blake’s adult children, Lowell, 58, and Vanessa, 56, who had cared for her and who would later die, were also there, Beverly recalled.

“We thought it was just an elderly person who passed away,” he said. “If only we had known then what caused her death, the children would be alive.”

A third child, Elaine, 51, who lived with her mother and had been her main caregiver, remains hospitalized, but she is improving

On Wednesday, a sister of Ruth Blake’s was taken to MedStar Washington Hospital Center for evaluation, a spokeswoman said. She has a fever, but does not appear to have other flu-like symptoms.

Blake’s son and her older daughter died within hours of each other Monday after they were hospitalized with upper respiratory symptoms.

Hospital officials said tests confirmed that the siblings had a strain of the influenza A virus that is circulating this flu season, and each also acquired a serious staph infection before being hospitalized.

The two developed severe bacterial pneumonia, which is not uncommon in people infected with the flu virus. The flu virus weakens and damages the lungs, making the person more susceptible to bacterial infections.

Calvert health officials said in a statement Wednesday that the cases were isolated to a single family and that “there are currently no other affected individuals.” Local health-care providers, they said, are not reporting any significant increase in patients with flulike symptoms.

David Rogers, the county’s health officer, said health officials suspect that Blake also had the flu and then suffered a serious lung infection that turned into pneumonia.

“In older people, that can often be fatal,” he said.

Blake had a flu shot, he said. None of the others were vaccinated.

What’s unusual, he said, is that the infection spread from the mother to three children, probably at her bedside.

Most likely, the mother’s coughing spread the virulent organisms into the air, and her caregivers, two of whom also had the flu, breathed them in and became infected, he said.

Rogers said the staph infection was probably one of the most virulent strains, methicillin-resistant Staphylococcus aureus (MRSA).

MRSA is a strain of the bacterium normally treated with common antibiotics in the penicillin family, such as methicillin. Antibiotic-resistant strains of the organism, however, have been turning up increasingly in places outside health-care settings.

“If you get pneumonia with a MRSA staph organism, which we think is what happened here, and if it was transmitted to somebody else, that organism is almost impossible to treat with antibiotics,” he said.

He said it was important to emphasize that the illnesses were isolated to one family and that family members were in close contact. “It is not a cause for widespread community alarm,” he said.

Although health department staff members wearing biohazard suits were inside the house this week, Rogers said they were taking routine precautions that were put in place after the anthrax scare in 2001.

But the public attention and the fear unleashed by initial reports of a “mystery illness” had put off many Blake family members. The family’s designated spokesman, Ruth’s son Leroy Blake, declined to comment for this story.

When Ruth Blake and her two children died, the extended Blake family was still absorbing another loss. Earlier this month, a family member was killed in an accident involving a fallen tree.

“It’s just been a lot,” said the Rev. Faith Lewis, pastor of Olivet United Methodist Church. “It’s been really hard for everybody.”

All was quiet again Wednesday at Ruth Blake’s house after being overrun by television news crews, health department officials in biohazard suits and members of the county sheriff’s special operations team, who took out mattresses and bedding as a favor to the family.

“My prayers are with the family,” one family friend wrote in the guest book for Ruth Blake that has been set up online through the Sewell Funeral Home. “God bless you and give you strength to go through each day in this sad and difficult time.”

Staff researcher Magda Jean-Louis contributed to this story.