Jeremy N. Morris, a British epidemiologist whose comparison of heart-attack rates among double-decker bus drivers and conductors in London in the late 1940s and early '50s laid the scientific groundwork for the modern aerobics movement, died Oct. 28 in Hampstead, London. He was 99 ½.

``He always insisted on adding the ½,'' said his daughter, Julie Zalewska.

The cause of death, she said, was pneumonia and kidney failure.

It had long been surmised that exercise and a healthy heart were correlated.

``You can go back to ancient physicians and philosophers like Hippocrates and Siddhartha who said exercise is good for you, but they didn't have any data,'' Steven N. Blair, a professor of exercise science and epidemiology at the University of South Carolina, said in an interview Thursday. ``Jerry was the guy who did the systematic research that invented the whole field of physical activity epidemiology.''

``His impact was huge,'' Blair added.

Terence Kavanagh, an internist and professor of exercise science at the University of Toronto, agreed, saying, ``The work he did set the tone for future research.''

Morris surmised that the proof could be found on the stairs of those double-decker buses. In 1949, he began tracing the heart-attack rates of hundreds of drivers and conductors. The drivers sat for 90 percent of their shifts; the conductors climbed about 600 stairs each working day. Morris' data, published in 1953, indicated that the conductors had fewer than half the heart attacks of their sedentary colleagues.

In a follow-up study, Morris found that a lower incidence of heart attack among people doing physical work was not, for the most part, related to other factors, like body type. Transport for London, the city's transportation agency, provided him with the sizes of the trousers it supplied to its workers. His data indicated that the conductors' waistbands were smaller, but that their protection against heart attack could not be explained by their relative leanness. They had a lower risk of heart attack whether they were slim, average size or portly.

To corroborate his findings further, Morris did a study of postal workers. Comparing those who delivered the mail by walking or riding bicycles with the clerks behind the window at the post office and the telephone operators, he found that the deliverers also had a far lower risk of heart attack.

Then, in the 1960s, Morris conducted an eight-year study of the overall physical activity of 18,000 men in sedentary civil service jobs. The data showed that those who engaged in regular aerobic exercise -- fast walking, cycling, swimming or other sports -- reduced their risk of heart attack by half.

In 1972, in Atlanta, Morris and Dr. Ralph S. Paffenbarger Jr. were awarded the first International Olympic Committee Medal in sports science.

Jeremy Noah Morris was born in Liverpool on May 6, 1910.

Almost every day, well into his mid-90s, Morris swam, pedaled his exercise bike or walked for at least half an hour.