What's most noticeable about the steady stream of patients into Charles F. Hoesch's medical office in the Baltimore suburb of Perry Hall is how few are actually sick.

There's Connie Garriques, 63, back from a vacation in New Hampshire, who's checking the blood pressure readings she takes at home against the ones recorded in the office. A 37-year-old landscape contractor named Brian Flick is getting his blood sugar and cholesterol tested and making plans for a procedure that will screen for colon cancer, the disease that killed his mother. John Davis, a 60-year-old engineer, is getting instruction on what to do about a slight elevation in blood pressure that, a generation ago, barely would have provoked notice.

Davis doesn't have heart disease, kidney problems or any of the other complications of high blood pressure. Nevertheless, there's no doubt in his mind -- or his doctor's -- that this is a problem worth addressing.

He and Hoesch come up with a plan of diet and exercise they hope will make a prescription drug unnecessary. In fact, those changes may improve not only his blood pressure but also his HDL -- the "good cholesterol" -- which frankly could use a little raising.

"See you in a couple of months," says Hoesch, bidding his patient goodbye.

This encounter, so ordinary in every way, bespeaks a sea-change in medicine in recent years.

Medical science and patient expectation are revolutionizing the concept of disease. No longer does disease denote pain, bodily damage or disability. Risk itself is now a disease -- or something very close to it.

In the last decade, adulthood's most common medical conditions -- high blood pressure, too much weight, elevated cholesterol, diabetes and osteoporosis -- have been redefined, or had treatment guidelines changed, in ways that increase every American's chance of being labeled as "diseased" and prescribed a drug.

Today 20 percent of all visits to the doctor in the United States are for one of those conditions. "Risk reduction" is American medicine's biggest growth sector and is likely to remain so. The list of possible "abnormalities" in the human body -- and the list of ways to correct them -- grows every day.

This trend comes at an ironic time. Life expectancy in the United States -- 76.7 years -- is at an all-time high. Mortality from the nation's three leading causes of death -- heart disease, cancer and stroke -- is in decline. The number of people over the age of 100 doubled this decade.

Of course, American medicine's emphasis on prevention is one of the main reasons for the nation's remarkable good health. Nevertheless, some people worry the trend may have hazards of its own.

"When the majority of the population is `sick' under some definition, it raises the fundamental question: What is health? What is being well?" said Lisa M. Schwartz, a physician at Dartmouth Medical School and the White River Junction (Vt.) Veterans Hospital. "It makes you wonder whether we've lost the concept of health."

When people take drugs (such as blood-pressure medicine) to lower risk for diseases they don't have (such as heart disease), only a small number ever benefit. Typically, it's one in 10, or fewer. That's because even though they may be at increased risk, their actual chance of getting the disease in question is quite small, with or without the preventive therapy. Benefit is further diluted by the risk of side effects, which occur with virtually every medical therapy.

"I think there will eventually be a backlash against the notion that we are all at risk for some kind of disease," said Robert Aronowitz, a physician and medical historian at the University of Pennsylvania. "I think people will come to realize there can be real harm in the diagnostic test that leads to the unnecessary surgery. They will see the `walking wounded' where previously they saw the `worried well' -- or perhaps the `not worried at all.' "

Research suggests there may be significant unintended effects of getting a medical diagnosis -- especially for a disease one can't feel.

Several studies have found that both mental stress and workplace absenteeism tend to rise after people are told they have high blood pressure. One-quarter of women screened and found to have osteoporosis chose to limit daily activities such as grocery shopping, a decision that "paradoxically . . . might accelerate bone loss and even increase the risk for fracture," wrote the authors of another study.

In its effects on public policy, the ever-widening umbrella of disease has already begun to test the seams of American medicine. Last summer's debate over whether the federal Medicare program should pay for prescription drugs was, at root, a debate over which definitions of disease should prevail -- those of 1965 or those of 1999.

When Medicare was created 34 years ago, nobody imagined a healthy 65-year-old woman might take $225 worth of drugs per month -- say, for hypertension, high cholesterol, osteoporosis and menopause -- and then live to 83 (the actuarial prediction for women who reach 65). Now, such a scenario is being played out by millions of people.

The expansion of disease isn't the result of human error or bad planning. It isn't like "grade inflation" in higher education or "bracket creep" in the tax code. It's the unavoidable product of scientific progress.

"Better epidemiological data has allowed us to move from calling something `just normal variation' to `risk factor' to `disease,' " said John Eisenberg, head of the federal government's Agency for Health Care Policy and Research (AHCPR). "But actually nothing has happened. The human body is the same."

In just the most recent example, there's now evidence a person's heart attack risk rises as the amount of homocysteine, an amino acid, goes up in their bloodstream. A few years ago, no doctor thought twice about homocysteine. Now, it's a broad new target for pharmacological therapy.

Technology is also helping to drive the redefinition of disease.

As recently as a decade ago, osteoporosis was a disease that almost always produced symptoms -- often a fractured bone -- before it was diagnosed. Today, painless "densitometers" can determine the strength of bones. About 20 percent of 65-year-old women -- and half of all 80-year-olds -- now meet the "diagnostic criteria" for osteoporosis, even though most will never break a bone.

On scientific grounds, it's hard to argue with disease inflation.

The NIH's Heart, Lung and Blood Institute last year published new cutoff points for overweight and obesity. They weren't picked arbitrarily. Instead, they marked the weights at which a person's risk of heart disease begins to rise steeply.

As it happens, 55 percent of adult Americans now fall into that danger zone. Under the previous (1980) cutoffs, only 26 percent of people were "overweight or obese." But the older definition had been set without an eye on actual health effects.

Better knowledge of where on the continuum of blood sugar measurements things start to get dangerous was, in part, behind a new definition of diabetes announced last year by the American Diabetes Association. Similar insights drove a redefinition of high blood pressure in 1977, which overnight changed the prevalence of the disease from 20 percent of the American population to 32 percent.

With some physiological variables, however, being "average" or "normal" may itself be a condition amenable to treatment. This makes it hard to draw the right border between health and disease -- if, in fact, there is one.

A dramatic example occurred last year when researchers announced the results of a study in which 6,600 middle-aged Texans with normal levels of cholesterol -- but slightly low levels of the desirable HDL -- were randomly assigned to take either a placebo or the cholesterol-lowering drug lovastatin.

After five years, there were 183 heart attacks or episodes of angina among the 3,300 people taking the placebo.

That's a very small number, although it wasn't surprising, because the people entering the study had no hint of heart disease, and normal cholesterol levels. Nevertheless, the drug-treated group did better. It had only 116 heart attacks.

"Who qualifies for drug therapy is an interesting question in light of these data," said James Cleeman, coordinator of the federal government's National Cholesterol Education Program (NCEP), which since 1988 has formulated advice to doctors and patients. "Things beside being able to say `there is benefit' come into play. How big is the benefit? How much does it cost? How long does it take?"

This much is clear: the risk vs. benefit calculations of preventive medicine are getting more complicated every year.

You have to treat 49 middle-aged people with average cholesterol to prevent one heart attack. Is this worth it? It costs $86,000 to save a year of life in a group of hypertensive 60-year-olds who are treated to get their blood pressure into the "normal" range. Is that worth it?

As both the universe of disease and medical intervention grow, those questions will have to be answered. How -- and by whom -- is uncertain.

"In the absence of top-down decision-making, it will probably take place at the pocketbook, at the hospital, at the health plan," said Eisenberg, of AHCPR. "They are the places where the dilemma cannot be avoided."