Forty-five juveniles and 11 adults have been isolated since Monday night after a 16-year-old at Cedar Knoll, the District of Columbia's juvenile detention facility in Laurel, was diagnosed as having bacterial meningitis.

The isolation and a precautionary treatment of the 56 youths and adults with antibiotics were scheduled to end this morning, according to Dr. Loretta Gilmore, chief of health services at Cedar Knoll.

Such a period of isolation is not considered essential in such cases, but District health officials said they decided on the isolation and medication because of the close quarters in which Cedar Knoll residents live.

The case at Cedar Knoll is the District's first this year. Last year, there were two reported cases of the disease, which attacks the central nervous system. There was one reported case in 1977.

Dr. Martin Levy, the District's chief of disease control, said, "We aren't too concerned about it because the people at [the federal Center for Disease Control in Atlanta] aren't too concerned about it."

Prior to the availability of antibiotics, bacterial meningtis proved fatal about 50 percent of the time. Today, however, 90 percent or more of its victims survive, although some suffer permanent damage to the central nervous system.

According to officials at D.C. Genral Hospital, where the Cedar Knoll resident was taken Monday, the youth is now listed in fair condition and was moved Wednesday from the intensive care unit to a general medical floor.

According to Gilmore, the youth on Sunday had early symptoms -- sore throat and a little fever -- which could be caused by any number of illnesses. "Monday he developed a higher fever and lethargy," she said. "As soon as the lethargy showed up, he was transferred."

Gilmore said his fever reached 103 to 10j degrees.

According to Cedar Knoll director Rimsky Atkinson, D.C. Superior Court officials were notified Tuesday morning that the 45 youths, some of whom were due for court appearances, had been placed in isolation.

There were rumors around the court that health officials were trying to track down lawyers and others who had come in contact with the youths, but according to Levey, no such attempt was made.

According to the book "The Control of Communicable Diseases In Man," which is considered a bible on the subject, the identification of contacts is "impractical" in meningitis cases.

Meningitis is not what is normally thought of as a disease. Rather, it is a word used to describe a condition that can be caused by several different bacteria and viruses.

The Cedar Knoll youth, say officials, was infected by a bacteria called neisseria meningitidis, which causes a form of meningitis called meningococcus meningitis.

An estimated 15 percent to 25 percent of persons in the population carry the bacteria, but do not become ill from it. According to "Control of Communicable Diseases," it is more likely for a person to be infected by a carrier than to pick up the disease from someone who is ill with it.

The bacteria invade the lining of the brain and spinal cord, called the meninges -- hence the name menigits -- and cause the tissue to swell. This swelling can result in permanent damage to the brain and central nervous system.

Bacterial meningitis can be treated with antibiotics if caught in time. In contrast, there are no medications to treat viral meningitis, but the disease is usually less serious and runs its course without causing permanent damage.