The Calvert County investigation into the flu deaths of 3 family members in Lusby, Maryland, found two of them–a 58 year-old brother and his 56-year-old-sister died of serious lung infections, a complication of seasonal flu. The CDC already has confirmed the siblings died of influenza A. The strain is still unknown. Their 81-year-old mother, who was being cared for by her three children died at home Thursday of complications from the flu. The surviving sibling, a 51-year-old woman, remains in guarded condition at MedStar Washington Hospital Center in Washington, D.C., where she is responding to treatment. Dr. David Rogers, Health Officer of Calvert County, says the investigation into the flu deaths of the three family members in Lusby, Maryland, is continuing.  Rogers told CNN while unusual, these were isolated cases and the community at large should not be overly concerned or alarmed. \"This is extremely rare. I think what happened here is that we had an elderly woman who got flu, then she suffered a complication of flu, a severe bacterial pneumonia. Three kids came to provide bedside care and came in close contact. They may not only have caught flu from her but the serious lung infection that she had.\" Rogers said. \"This is a very isolated situation where you have four people in a home, in close contact. This happens not infrequently in elderly people.\" The Centers for Disease Control and Prevention estimates 90% of seasonal flu related deaths and more than 60% of flu related hospitalizations in the United States each year are in people 65 or older. The sister of the deceased 81-year old woman is now also being treated for flu like symptoms at MedStar Washington Hospital Center. Dr. Janis Orlowski, Senior Vice President and Chief Medical Officer at MedStar Washington Hospital Center, says the woman was admitted last night and is in fair condition. \"We don\'t yet know what this is about,\" Orlowski said. \"She has a fever and a cough. They were all together at a funeral last week. Individuals could easily have caught the flu–a large gathering of people, hugging, consoling, possibly sharing a meal.\" \"If the individuals had the flu at that time it is quite likely that others are showing signs of the flu,\" says Orlowski. \'If I was in that family and felt ill, I would seek medical attention early.\" Orlowski  also treated the brother, who passed away at the hospital Monday night. She says the bacterial infection they had was a staphylococcal (staph) infection, which she calls a super infection. Cultures on the surviving sister are not back yet. The source of the staph is still unknown, but Orlowski says the bacteria is naturally on the body. On  skin, in the nose. Most people, she says, don\'t catch it from someone else. \"The question is did the mother have the staph where it went to the hands of the children and they got infected or did each end up becoming super infected with their own bacteria and we don\'t know the answer to that. It\'s likely we will never know.\" And rare enough that they looked for other sources. \"We did look for poison, carbon monoxide in the house, did they all ingest something?\" Orlowski asks. \"It\'s astonishingly rare, such that we are all looking that other explanations.\" Orlowski says over the last several days the hospital emergency room filled with nearly 400 patients. Most had chronic illnesses like heart failure, chronic obstructive pulmonary disease (COPD) and liver disease made worse by a bout of flu. \"It\'s flu season in the mid-Atlantic area so I urge the public to get vaccinated if you have not been, wash your hands, and if you are ill, don\'t go to school or work. We don\'t want you to spread the illness.\"