Influenza hit the Washington area with unusual force during the last week, sending throngs of coughing children and adults to hospital emergency rooms and practically emptying classrooms at a few hard-hit schools.

"It hits a school -- bingo! -- and surrounding schools don't seem to be affected," said Dr. Marjorie Hughes, chief of school health services in Arlington. "Yesterday was the first time I had any of my nurse calling in, saying, 'Wow.'"

The bug, which laboratory experts have identified as influenza A virus, has driven absentee rates at a few area schools to 20 percent or higher, rather than the normal average of 8 percent.

A spokeswoman for Walter Reed Hospital, which staffs military clinics in the area, said doctors had noted an increased number of cases that seemed like flu among military staff at the Pentagon, Fort Myer in Virginia and Fort McNair in the District. Workers at offices in various parts of Washington also said many of their colleagues had been sick.

"I walked in Monday morning and all our [infirmary] beds were filled," said Dr. Eugene Zimmerman, man, acting medical director of the student health service at Gallaudet College, which reported the highest rate of illness in the city with a third of both students and faculty out sick. "It looks like the flu: the runny nose, the achey muscles, the headache and cough, cough, cough."

He said the illness had stricken so many of the school's 375 students that, in addition to the 25 beds in the infirmary, the school had been forced to keep an entire dormitory open during the day for about 75 other patients.

Emergency room officials at hospitals throughout the area reported that in the last week they had been seeing more and more people for what looked like flu: cold symptoms and cough plus a fever that sometimes lasts for several days. "Today we had three whole families with it -- children, mother, older sisters, et cetera," said Mary Gavin, head nurse at Providence Hospital emergency room.

The Children's Hospital emergency room has been treating 50 percent more patients than usual, with flu symptoms accounting for the increase, according to Dr. Daniel Ochsenschlager, the department's director. "This weekend was really bad," he said. "I think it's going to be bad for quite a while."

Testing at the hospital's virology laboratory has revealed that all the flu cases identified so far are caused by an influenza A virus called H3N2, of a strain that has traveled around the world since it was first isolated in Thailand in 1979. The virus differs from last year's predominant flu bug, which was influenza B, but those who received doses of 1980 flu vaccine in the last few months will have substantial protection from it, according to Dr. Peter Patriarca, a medical epidemiologist at the Center for Disease Control.

Patriarca said flu shots are not necessary for everyone, but are recommended for anyone who is over 65 or who has a chronic illness such as heart or lung disease, diabetes or kidney failure. He said patients develop immunity about two weeks after being vaccinated. Vaccination is particularly effective for preventing flu outbreaks in hospitals or nursing homes.

He said the type A flu virus included in this year's vaccine is not identical to the strain showing up in patients, but is closely enough related to provide 60- to 70-percent protection to those who receive shots. "If they do get exposed, they might have a very mild case," he said.

He added that a drug active against viruses, amantadine, can prevent influenza if taken in advance, but that the medication's potential side effects -- stomach upset, dizziness and drowsiness -- can make the treatment worse than the disease for many patients.

Tests on this year's influenza virus show that it is very similar to the strain that caused much of the area's flu during the winter of 1977-78, which is reassuring, since it means this year's bug is less likely to spawn a major epidemic, according to Carl Brandt, a research virologist at Children's Hospital.

"The one you'd really worry about is [a virus whose] surface pattern is markedly different," indicating that the virus has changed so much the few people are resistant to it from previous exposures. Of seven patients hospitalized there for severe illness from influenza since early November, five were babies under 6 months old, too young to have any resistance from previous bouts of flu.

"Kids who weren't born when the virus was last around would be the most susceptible," he said.

Doctors interviewed said flu can be expected to produce cough, sore throat, head and muscle aches, runny nose and fever of 101 degrees or higher, often lasting several days. Although vomiting and diarrhea are not typical of flu, they said many children they are seeing do have these symptoms. Most cases are mild enough to be treated at home with bed rest, fluids and medications such as aspirin or acetaminophen (Tylenol) to keep fever down.

In some patients, especially very young children or the elderly, illness can be more severe. In particular, a person should be checked by a doctor if he has shortness of breath, a very high fever or severe cough, or if he cannot eat or keep fluids down.