A 14-year-old-student at North Bethesda Junior High School is recovering from polio that she contracted last month from her newly vaccinated 4-month-old niece because she herself had not been fully immunized.

The student, a ninth grader on the girls' basketball team, was infected with live polio virus "shed" in the baby's stool shortly after the infant received a dose of polio vaccine, according to Allen Swick, the school principal. She was hospitalized for about two weeks with muscle weakness of one arm and both legs.

Last week when tests confirmed the student had polio, parents of the 466 students at the school were advised to make certain their children's immunizations were up to date, Swick said.However, health officials said there was virtually no chance that the student could have infected others with the disease.

Swick said the girl has been discharged from the hospital and is receiving physical therapy. "There was a mild amount of paralysis . . . in one arm, and some weakness in the legs," he said. He added that doctors are optimistic that she will recover.

According to Dr. Jonathan E. Kaplan, an epidemiologist at the federal Centers for Disease Control familiar with the case, the girl's illness began in mid-January after she had baby-sat for her infant niece. After suffering two days from what seemed like the flu -- headache, sore throat, muscle aches and fever -- she noticed that one of her arms was weak. Kaplan said she entered a hospital on Jan. 17 and was discharged two days later, but became worse at home and reentered the hospital on Jan. 20.

He said doctors will not know for 60 days whether she has been permanently affected. "In paralytic polio, frequently the victim is left with a permanent . . . weakness of one arm, one leg or some atrophy [shrinkage] of muscles," he said. "There's a reasonable chance that will happen."

Polio -- or poliomyelitis -- is caused by a virus that can damage or destroy the nerves that control muscles, causing weakness or total paralysis. Until 1954, when Dr. Jonas Salk perfected the first effective vaccine, there were about 20,000 cases annually in the United States.

The oral polio vaccine now used in this country is made from live polio virus that has been "attenuated" or weakened in the laboratory, so that it will be "strong enough to cause an immune response but not strong enough to cause an infection," Kaplan said.

However, occasionally infections occur -- either in the recipient of the vaccine or in nonimmune people who come in close contact with the recipient, he said. The virus is normally shed in a recipient's stool for four to 30 days after the first dose is given.

He said the girl had moved frequently during her childhood, and apparently had received only one dose of polio vaccine. Although 65 percent of recipients develop immunity after one dose, current health regulations require four doses -- usually given at the ages of two, four and 18 months, with a booster before school entry, he said.

About two-thirds of American children have received three doses, and more than 90 percent have received at least one dose, he said. A Montgomery County health department spokesman said 98 percent of county schoolchildren have been vaccinated against polio.

Nine cases of polio occurred in the United States during 1980, according to the Centers for Disease Control. There were 34 cases in 1979, the year of a polio outbreak in the Amish community in Pennsylvania and several other states among people who had declined vaccination for religious reasons.

As vaccination programs have drastically reduced the number of cases of natural or "wild-type" polio infections, an increasing proportion of the cases that occur in the United States are vaccine-related. Of the 186 cases reported from 1969 to 1979, 23 occurred in vaccine recipients and 50 in persons infected by vaccine recipients, according to a CDC report.

The risk of acquiring vaccine-related polio is very slight -- about one case per million doses given.