NEW YORK -- A reclusive little woodtick a quarter-inch long and a corkscrew-shaped microbe thousands of times smaller have attracted scientists from three continents to a meeting here on the implications of a rapidly spreading public health menace.

Found only on offshore islands near Cape Cod less than two decades ago, Lyme disease has now been seen on every continent except Antarctica. Although not officially reportable to the federal Centers for Disease Control, Lyme is recognized today as more common than the better known, and reportable, Rocky Mountain spotted fever.

"By all appearances, Lyme is the most prevalent tick-borne disease in the U.S.," epidemiologist Lee Harrison of the CDC's special pathogens branch, said in a telephone interview. Harrison indicated that known cases of Lyme now outnumber cases of spotted fever almost two to one.

The importance with which this relatively new disease is regarded can be gauged by the fact that scientists from Austria, China, Denmark, France, West Germany, Britain, Norway, the Soviet Union, Sweden, Switzerland and the U.S. are presenting papers at the three-day meeting that started yesterday.

The International Conference on Lyme Disease and Related Disorders is the third in a biennial series that began at Yale University in 1983. The second was in Vienna, Austria, in 1985. Current interest in Lyme disease dates from 1975, when an arthritis-like outbreak struck children in Lyme, Conn., where the disease got its name.

It was first called "Lyme arthritis" -- a name that, to some degree, has stuck -- but in the last dozen years it has become apparent that the disease has many more aspects. Like syphilis at the turn of the century, Lyme is being referred to as "the great imitator" because of its ability to mimic other diseases. And, like syphilis, Lyme is caused by a spirochete-type micro-organism.

The Lyme spirochete, Borrelia burgdorferi, recently has been associated with symptoms similar to multiple sclerosis, amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease), and Alzheimer's disease, a mind-destroying ailment that recently killed film star Rita Hayworth.

The Lyme germ has been isolated from brain tissue of patients who died after being diagnosed with Alzheimer's, said pathologist Alan B. MacDonald of Southampton, N.Y., where Lyme disease incidence is high.

MacDonald and Dr. Burton A. Waisbren, an internist from Milwaukee, have independently linked Lyme and ALS, leading MacDonald to initiate a study that will test people with Lou Gehrig's disease for antibodies to the Lyme spirochete.

If some patients suffering these symptoms are found to have Lyme disease rather than ALS, they may be helped by medication. "ALS has no effective treatment," MacDonald said in a recent interview, "but Lyme can be treated with antibiotics."