When a doctor utters the diagnosis "multiple sclerosis," it is in most cases bad news.

Multiple sclerosis or MS -- the verdict given former hostage Richard Queen yesterday -- is a serious, usually progressive, often crippling nerve disease.

It is the most common nerve disorder of young men and women, with persons between the ages of 20 and 40 making up nearly 70 percent of the victims.

To say MS is "usually progressive" does not mean it is always so, however. And it does not mean that many, even most, patients cannot lead normal or nearly normal lives with good fortune and modern care.

The disease acts by destroying myelin, the fatty material that sheathes nerves.

Although the disease usually has an unpredictable, up-and-down course, nerves too may be destroyed in time.

The effect in any case is to distort the flow of nerve impulses, just as damage to the insulation of a telephone cable may distort its messages.

The cause of the disease is unknown. One new theory is taht it may at least sometimes be a long, slow response to virus infection, possibly measles infection in childhood. But the virus might only cause MS in genetically susceptible persons.

For some reason, perhaps virus distribution, the disease is most common in northern and temperate climates. "But the virus theory is a promising lead and no more," Dr. Thomas Chase of the National Institute of Neurological and Communicative Disorders said yesterday.

There are several other suggestions for a possible cause. Among them: some disorder of immunity; a disorder of the body's enzymes; some response to excesses or deficiencies in diet; a reponse to physical trauma or abuse.

Among other factors that may bring the disease to the surface or make it worse may be the type of emotional stress any long-term hostages would suffer.

But all conjecture about MS's cause or causes is only conjecture, no more, Chase and other doctors report.

There is no question, Chase said, that MS is "a major cause of serious neurologic disability," and "in many cases the outlook is not optimistic."

But there can also be mild cases, and other doctors stressed the optimistic side. "Most patients, probably the majority, have moderate cause with only modest disability," said Dr. Stanley Cohan of Georgetown University.

Dr. Robert Slater of the National Multiple Sclerosis Society said, "About a third of all patients are very mildy affected and may be completely without symptoms between relapses. Another third may have a more lasting effect after each relapse. A third may have great difficulty and even become bedridden."

Life expectancy varies. One current medical text says it ranges from "a few weeks to over 50 years." Slater said life span is "probably 85 percent of normal, on the average."

Life expectancy has been lengthened greatly in the last 10 to 20 years by better treatment of the bladder and lung infections some patients suffer.