A German measles epidemic swept the United States about 18 years ago, and its 20,000 victims are now inundating the limited facilities of the nation's colleges for the deaf, causing some qualified students to be turned away.

Doctors call it the "rubella bulge," the two-year epidemic of German measles in 1964 and 1965 that was the most severe ever recorded in the nation. It came a few years before a vaccine was developed to immunize pregnant women against rubella, and it resulted in almost three times the number of infants who are normally born deaf each year.

"The rubella epidemic of 1964 and '65 was a national tragedy that went largely unnoticed," said Dr. Edward C. Merrill Jr., president of Gallaudet College. "Unfortunately, it is about to become noticed at a time of economic stress when there is a national need to economize."

The enrollment at Gallaudet College, the federally supported institution for the deaf in Washington, will rise from 1,500 to almost 2,000 next year and will climb substantially the year after. For the first time in its history, Gallaudet will have to turn down qualified deaf applicants because it simply doesn't have the room.

So tight is floor space at Gallaudet that Congress earlier this year gave it the entire campus of Marjorie Webster Junior College, which closed in 1971 because of declining enrollments. The federal government acquired Webster a few years ago to turn into a national fire fighting academy.

"We would never be able to handle next year's class without this gift. Congress has been extraordinarily generous with us," Merrill said.

And Dr. William E. Castle, director of the National Technical Institute for the Deaf in Rochester, N.Y., said, "We are about to deal with more than twice the number of first-year students we've ever had. We don't know what to expect because we've never dealt with anything like it."

Castle said enrollment at his institute will jump from its present 986 to more than 1,250 next fall, the most students the institute has ever had. Castle said he had to ask Congress for $3 million (which Congress appropriated) to construct a new academic building to handle the incoming class, and the Rochester Institute of Technology (where the institute is located) for dormitory space to house the class.

Rubella in pregnant women was not recognized as a cause of birth defects until the 1940s, when a search began to find a vaccine to prevent the disease. Doctors quickly discovered that the rubella virus can infect a developing fetus through the placenta crippling the still-developing ears, and in a smaller number of cases the eyes and brain.

"The rubella virus deranges the blood vessels in the placenta and ends up attacking specific cells in the fetus," said the Food and Drug Administration's Dr. Paul Parkman, who in 1965 with Dr. Harry M. Meyer Jr. developed the first rubella vaccine when they were both at the National Institutes of Health. "It manifests itself in the eye and the ear by causing an unusual amount of deafness and congenital cataracts in the newborn afflicted with the virus."

The Parkman-Meyer vaccine turned that all around. The vaccine was licensed in 1969 in the United States and has been given to more than 100 million Americans. The Centers for Disease Control in Atlanta estimate that the country suffers somewhere between 35 and 100 congenital rubella cases a year right now, down from an average of 7,500 a year before the epidemic of 1964 and 1965.

"Our aim for the vaccine was to have no more epidemics," said Dr. Alan Himman of the CDC. "We've done that and more."

Fully half the victims of what someday may be called the "last great rubella epidemic" are on the threshold of a college career, enrolling not only at the Rochester institute and Gallaudet but at California State University at Northridge, Southern Illinois University and Madonna College in Michigan that accommodate the deaf. Many rubella victims never make it to college because they are blind or brain-damaged as well as being deaf.

Even so, many of the deaf rubella victims about to enter college are otherwise handicapped. Many have bad eyesight, usually because of cataracts suffered when they were in the womb. Others have heart defects and kidney ailments and many contract diabetes before they reach the age of 21.

This will make it even more difficult for the colleges like Gallaudet that are readying themselves for the largest incoming classes in their history. Almost all of the increased students are rubella victims, meaning their classrooms will have to be better-lit, their counseling more intense and their medical care more complete.

"Our hardest job will be to keep the size of the classes small," said Raymond Trybus, Gallaudet's counselor. "If you're 15 rows back and can't follow the teacher's sign language, there's not much point in going to school. Unfortunately, a better sound system won't help us like it would in school situations where deafness is not involved."