Health investigators concluded today that a third child, a West Warwick middle schooler, suffered from encephalitis probably brought on by a common infection that usually causes pneumonia, deepening a medical mystery that has drawn the attention of federal medical authorities.

The bacteria that seems to have caused the infection, mycoplasma pneumoniae, very rarely causes encephalitis, and encephalitis itself is extremely rare, so the cluster of three cases -- which included one death -- has prompted an intensive investigation by the U.S. Centers for Disease Control and Prevention. The surviving children have recovered.

Five doctors from the federal agency are in Rhode Island talking with school nurses and the families of children who have been out of school, trying to identify any additional cases. One possibility under consideration is that a new, more virulent strain of the bacteria may have developed, according to Dr. David R. Gifford, Rhode Island director of health.

Mycoplasma pneumoniae is a common bacteria that spreads much like a cold and usually manifests itself as bronchitis, or as walking pneumonia. In what health officials say are “very, very rare” cases, people who contract the bacteria can develop meningitis, an inflammation around the brain, or encephalitis.

Anyone with symptoms of walking pneumonia should seek treatment, health officials say. Symptoms include fever, cough and shortness of breath. Any symptoms of meningitis or encephalitis should prompt urgent medical evaluation, according to the Health Department. Such symptoms include moderate to severe headaches, disorientation or confusion, fever and neck stiffness.

Gifford met with 40 to 50 parents this morning on the campus of Deering and the West Warwick High School. After the meeting, Gifford characterized the level of concern among parents at the meeting as "appropriate."

He told them that good hygiene procedures, such as regular hand washing, and covering up while coughing is the best way to prevent the spread of the mycoplasma bacteria. Children in schools are no longer told to cover their mouths with their hands, but rather the common advice now is to sneeze and cough into the crook of one's elbow.

So far, health officials do not know of any children currently ill with mycoplasma or with encephalitis. All the eight known mycoplasma cases -- which include five children who had pneumonia without encephalitis -- occurred in November and December.

Seven of the eight infections were in children who attended the Greenwood Elementary School in Warwick. The Health Department has closed the school for this week, and has offered antibiotics to the families every student. Nearly all agreed to take the drugs.

The Health Department distributed 1,182 doses of antibiotics on Sunday, yesterday and today. The department ordered the drugs from a distributor at a cost of $57,000, charged to a Health Department credit card. Several children had upset stomachs, a common reaction to the medication, but no allergic reactions were reported.

Health officials have asked all schools in the state to examine their absentee records and report back to the department if they believe their students have had excessive respiratory illnesses lately.

If a school believes they have had an excessive amount of respiratory illnesses, the Health Department and CDC specialists will investigate further, according to Dr. Robert Crausman, chief administrative officer of the state Board of Medical Licensure and Discipline.

Also today, health officials continue to distribute written questions to parents to monitor whether their children have been sick recently.

In West Warwick, the questionnaires ask if children have been to the doctor for sickness since Oct. 1. If yes, parents are asked to list their symptoms and whether the children had chest X-rays, were given antibiotics and were diagnosed with pneumonia.

Also, state health officials have established a toll-free information line at (800) 942-7434 for parents of children in Warwick, West Warwick and Coventry schools. People can also visit the Health Department's Web site for more information.