The Marine Corps captain who was hospitalized 86 days at Walter Reed Army Medical Center with a suspected case of rabies was released and "left the hospital walking under his own power" last week, a hospital spokesman said last night.

"He came absolutely as close to dying with an infectious disease as you can come and not die," Dr. Kenneth W. Bernard, a medical epidemiologist with the Centers for Disease Control in Atlanta and the leading authority on rabies, said about the 34-year-old officer.

Medical authorities now say that they have "no idea" what caused the illness in the Marine, who--about a month after he entered the hospital--required a respirator to breathe and a dialysis machine to cleanse his blood, was semiconscious, had limited movement in his limbs and suffered from an inflammation of the brain. None of the tests for rabies and a host of other viral infections has proved positive.

The name of the Marine, a native of Las Vegas who has been stationed at Quantico since July 8, 1980, has not been released at the request of his family. He is married and has two small children, according to Marine spokeswoman Capt. Patrice Mauk.

Prior to his release from the hospital Friday, the Marine had visited his family on several weekends. During the first weeks of his illness, his arms and legs were almost totally paralyzed; now he needs only a cane to get about.

Whether the captain will be able to return to active duty will depend upon the degree of rehabilitation, authorities said.

He was placed on the Marines' "temporary disability retired list" on March 18, Mauk said. Military officials explained that under the retirement, the Marine's family would have received better benefits had he not survived.

Military authorities said that the captain would have to petition for a medical board review--probably within a year--if he wants to return to active duty.

If his rate of recovery is favorable, they said, he could be maintained on the temporarily retired list. If he does not recover sufficiently to return to active duty, he would be retired permanently, they said. While he is temporarily retired, the Marine receives a pension, authorities said.

"Just because we don't know the cause of his illness at this time doesn't mean that people have stopped looking," Bernard said last night.

"We know that we had a neurological disease that was probably caused by a virus," Bernard said, adding that the lack of a positive lab test made it "highly unlikely" that the Marine had rabies.

"Throughout his illness, he had a disease compatible with rabies," he said. "We don't know what it is so we can't say what it isn't until we know what it is."