Georgetown University announced yesterday it has shut down its main campus until Tuesday as more than 50 percent of the 7,367 graduate and undergraduate students were absent from classes yesterday, all presumed to be victims of Russian flu.

The school has not closed its medical, dental, nursing or law schools, nor its continuing education classes.

While physicians have not been taking throat cultures of all the students, most of whom are being treated at the school's infirmary and the Georgetown Hospital emergency room, Russian is now the prevalent flu strain in Washington.

Most of the ailing students are remaining in their rooms, said university spokeswoman Anne Marie Ellis. She said the student health service is providing aspirin in the dormitories and belivering fruit juices and sodas to sudent quaters twice a day.

The virus also has struck at the U.S. Naval ACademy at Annapolis, where more than 75 percent of the 4,3000 mudshipmen contracted the virus, and at George Washington, Howard and Maryland universities. All those insitutions, however, continued to operate despite the illness.

The outbreak of the latest strain of influenza has markedly increased absenteeism in the District's public school, and in scattered suburban schools as well. District epidemiologist Martin E. Lcvy has said that he would not be surprised to see the absenteeism rate in schools go as high as 50 percent. It is close to 20 percent in some schools now.

At Montgomery County's Seneca Valley High School, about one-third of the school's 1,340 students were absent Wednesday. And the absentee rate yesterday at Bailey Elementary School in Falls Church ran at 172 of 560 pupils. One second-grade classroom has 26 youngsters had only 10 yesterday.

Physicians at Children's Hospital National Medical Center reported that they are treating about double to 60 to 90 children they usually see in the emergency room each day.

The Russian flu, which is supplanting the A-Victoria and A-Texas strains prevalent a few weeks ago, is striking mostly those under 24 years of age.

Those between the ages of 24 and 50 have some immunity to the flu, which is similar to a strain of flu present in this country between 1947 and 1957.

Russian flu, like all flu strains, is an upper respiratory disease that causes fever, chill, head and body aches, sore throat and a cough. It generally lasts three to five days, and should be treated by bed rest, aspirin or an aspirin subsitute, nourishment and plenty of fluids.

Flu generally is dangerous only those suffering from chronic uper respiratory and cardiovascular conditions, the very young and the very old.

Dr, Robert Parrot, director of Children's Hospital and a nationally known expert on viruses, recommended yesterday that parents of children with chronic conditions consult their pediatricians about the possible use of Amantadine, a drug that can be used to prevent - or lessen the effects of - viral infection.

Parrott said that anyone running a fever of 104 or above, anyone having trouble breathing or a person with flu symptoms that persist for more than a week should consult his or her physician.

According to Parrott, gastrointestinal problems, such as diarrhea and nausea, which most persons associate with flu, are not flu symptoms. What one may be suffering from, he said, is a gastrointestinal virus and perhaps, a flu virus at the same time.

Parrott predicted that "within a couple of years, every one" in the susceptible age group "will become infected" with Russian flu.

He said that "this year, and this is a seat-of-the-pants guess, I'd say one third will get it."

Flu, said Dr. Joseph Bellanti, chief of immunology at Georgetown University Medical Center," is really a pain in the head. I remember the Asian flu. I had the worst headache of my life."

The head and body aches associated with flu are caused by a toxic substance that is part of the virus, said Bellanti, who said that the virus's first areas of attack are the nose, throat and lungs.

While flu usually can produce memorable headaches, coughs and sore throats, it usually leaves the victim none the worse.

One of the principal killers associated with flu is pneumonia, but since the advent of penicillin, pneumonia has ceased to be a major threat except to the elderly and infants.

In 1918 and 1919, however, pneumonia was a serious threat. The record-breaking world-wide flu panderaic of late 1918 and early 1919 killed 21 million persons, more than half a million in this country alone.

In fact, the Oct. 13 1918 Washington Post carried a headline, "Flu Under Control. 56 Deaths Yesterday . . .Malady Grips the Nations."