The viruses that cause such common diseases as mumps and German measles may also be causing diabetes, according to two important new discoveries.

Diabetes affects at least 5 million Americans - and perhaps 300 million persons throughout the world - and for years scientists have been trying is vain to discover what triggers it in some susceptible persons but not others.

Providing strong evidence that virus infection may be one such cause:

Scientists at the National Institutes of Health in Bethesda have successfully infected human pancreas cells - specifically the "beta" cells that make insulin - with mumps virus. The virus then kills the cells.

Australian scientists have shown that some children affected in the womb by their mothers' German measles later develop diabetes. And German measles' infection in rabbits, the same scientists say, causes changes in beta cells.

Defects in these cells prevent the body from producing enough insulin to burn up sugar, thus causing a form of diabetes - the juvenile-onset form, which accounts for about one case in 10.

The lives of the diabetics who develop the disease in childhood or youth are shortened by about a third, on the average, though with modern treatment many live to old age.

In adult-onset diabetes, there is still debate over whether the beta cells are abnormal. But in both the juvenile and adult-onset illnesses, there is too much unused sugar in the blood-stream, interfering with body chemistry and threatening many organs.

Since the turn of the century, there have been sporadic reports of diabetes developing after mumps, measles, polio, encephalitis, influenza and other virus diseases.

But only in recent years has there been any solid progress in investigating these clues.

Dr. John Craighead, now at the University of Vermont, first reported in 1968 that he had produced diabetes in mice by infecting them with the virus of a serious animal disease - encephalomyocarditis, or inflammation of the brain lining and heart.

At NIH, Dr. Abner Notkins further developed this demonstration that a virus could make animals diabetic.

But this virus does not cause human disease. The mumps virus does, and it is mumps infection that has most often been associated with diabetes in case histories.

This month an NIH group headed by Notkins, head of the Laboratory of Oral Medicine in the National Institute of Dental Research, reported infecting human pancreas cells with mumps virus and showing that the virus could reproduce itself in the crucial beta cells.

Dr. Gregory Prince, Dr. A. Bennett Jenson, Lloyd Billups and Notkins reported these results in the British scientific journal Nature.

In the same week, three Australians, Margaret Menser, Jill Forrest and Robyn Bransby of the Royal Alexandra Hospital for Children at Camperdown, New South Wales, reported their findings in the British medical journal Lancet.

They found that of 45 children born with congenital rubella (German measles) and first seen by them in 1967, nine have since developed diabetes. Among another 318 congenital rubella patients, they found eight more cases.

Notkins said last week that he is now doing research on other human viruses and diabetes, with what looks like promising results.

"We still don't have proof that viruses cause diabetes,"" he said. "We don't know whether the number of cases caused by viruses, if viruses are a cause, is large or small.

"What we do know is that we still have a lot to learn about this disease. We do know there are hereditary influences. My educated guess is that viruses will be proved to be one more specific cause. We may find many causes," he said.

The search for the knowledge is urgent, he said, for, "we still don't know how to prevent the long-term complications" that afflict many diabetics despite the wide use of insulin and other treatments.