Three members of a Maryland family died after contracting severe respiratory illnesses and a third family member is hospitalized in critical condition, the Calvert County Health Department announced Tuesday.

Officials are trying to identify the illness that killed an 81-year-old woman and two of her children, both in their 50s, who cared for her in her home in Lusby. Another of the woman's children is seriously ill at Washington Medical Center.

The elderly woman became ill around Feb. 23 and died on March 1, according to WJLA. The woman's son and daughter developed symptoms around Feb. 28 when they came to care for her. They died on Monday.

The health department announced that no one outside the family has fallen ill and they are not recommending special precautions at this time.

Although few details are available about the illness, infectious disease specialists speculate there could be a number of causes.

"The first thing that comes to mind is influenza. It can be devastating and make people very ill," said Dr. William Schaffner, chair of preventive medicine at Vanderbilt University Medical Center in Nashville. "But when they're a cluster like this, we have to wonder if it may be a mutant strain of flu virus. There's been some concern about a swine flu variant."

Pneumonia is also a possibility, as is Legionnaires' disease.

"Legionnaires' could cluster, and that would implicate perhaps something about this home environment," Schaffner added. "Bacteria like moisture, so there would need to be an investigation of the heating and air conditioning systems."

There could also be other environmental causes, such as a toxin, or even carbon monoxide poisoning.

Dr. Gio Baracco, associate professor of infectious diseases at the University of Miami's Miller School of Medicine, speculated, based on the very limited amount of information available, that the cause is non-infectious and could be something envirnonmental.

"The reason is that for most infections, transmission rate is not 100 percent and the fatality rate is not 100 percent," he said. If the woman's hospitalized daughter dies, the case fatality rate will be 100 percent, he explained.

"The fact that they're in the same household and they're not seeing the same thing going on in the community is unusual," he added.

An earlier press release from the health department said the woman's home is about a mile away from a power plant, but there has been no link between their proximity to the plant and the illness cluster.

The health department recommends that people regularly wash their hands and limit contact with anyone who is sick. They also urge anyone experiencing flu-like symptoms to seek medical attention.