Rasmussen's encephalitis, identified 30 years ago, remains a medical mystery.

"We still don't know very much about it," says Dr. Theodore Rasmussen, emeritus medical director of the Montreal Neurological Institute, who discovered the disease that bears his name. Rasmussen also treated 39 patients by hemispherectomy -- the type of surgery Maranda Francisco had -- before he retired in 1980.

No other medical center has treated as many patients with Rasmussen's, which is an extremely rare inflammation of the brain tissue. Only a handful of medical centers around the world have studied and treated such patients. Most neurologists won't see a case during their careers.

Scientists don't know what causes Rasmussen's encephalitis or, in most cases, how to treat it. In many ways it behaves like a virus, but unlike most viruses it develops slowly and progressively, sometimes over a period as long as 15 years, before burning itself out. And no one has been able to isolate a Rasmussen's virus.

Nothing can reverse the brain damage from Rasmussen's. But if the patient is young enough and the diagnosis is made early enough, radical surgery sometimes can eliminate the seizures and check the inflammation before it engulfs the entire brain.

"There really is no treatment," Rasmussen says. "What you can do is treat the seizures early in the game. The earlier it's done, the better."

One thing 4-year-old Maranda Francisco had going for her last month, when she underwent surgery to remove the inflamed left hemisphere of her brain, was her age. Her young brain was flexible enough so that the right hemisphere had already assumed many of the functions of the ravaged left hemisphere.

Hemispherectomy -- removal of half of the brain cortex -- is not a new operation. It was tried in the 1920s on patients with brain tumors, but was no more successful than radiation therapy. It was used to treat some kinds of epilepsy in the 1950s, then fell out of favor because of recurring long-term complications, such as toxic effects of iron buildup in the empty brain cavity left by the operation. Refinement of the surgery to reduce the risk of complications has brought about its revival as a treatment for some children with brain tumors or Rasmussen's.

Research continues in an effort to pinpoint the cause of Rasmussen's -- virus or nonvirus -- as well as other abnormalities of the human brain.

With her parents' permission, samples of the removed left hemisphere of Maranda's brain have been sent to leading epilepsy research centers around the world, including the National Institutes of Health.

"I don't take a day of my time lightly," says Dr. Roger Porter, chief of the medical neurology branch of the National Institute of Neurological and Communicative Disorders and Stroke. "But I went up there and stood in the operating room all day long just to get a sample of her brain tissue."