Human health is the amount of a person's ongoing capacity for physical, emotional, mental, and social adjustment to their surroundings.

There are a number of other definitions that could apply. Particularly, what is considered to be "excellent" health can differ greatly. A sea-level resident moving to a new home in the mountains, where the atmosphere has a lower content of oxygen, may experience shortness of breath and anemia until his or her red blood cell count adjusts itself to the altitude, or the relatively frail person who remains "well" within the ordinary environment of his or her existence may pass away from a heart attack from vigorous shoveling after a snowstorm. Therefore, even by this definition, the idea of good health must take environmental change into consideration.

Excellent health can be described as the absence of disease, whereas bad health can be defined as its existence. This is especially true with continuous disease, since a person experiencing a sudden episode of seasickness may not be considered to have lost their good health as a result of such an accident.

Truth be told, there is a huge range of variation between health and disease. It only takes a few instances to make the point: (1) A person's blood sugar level being high 15 to 20 minutes after a meal is biologically typical. However, if the sugar level is still elevated two hours later, this is abnormal and might be a sign of sickness. (2) A "healthy" person may have grown allergic to one particular drug, possibly as a young child. If the allergic individual never again comes into touch with the offending antigen and all other circumstances remain the same, they will continue to experience their current level of health. The person may experience anything from a moderate allergic reaction—a simple rash—to severe anaphylactic shock, coma, or even death, depending on the circumstances—should they come into touch with that allergen again, even 20 or 30 years later. Thus, it is clear that, in contrast to disease, which is typically recognizable, tactile, and relatively easy to define, health is a condition that is rather ambiguous and challenging to define.

Furthermore, the terms "health" and "physical condition" are not interchangeable. A basketball player who is seven feet tall may be in outstanding physical shape (despite being taller than average), but their health varies based on factors like if they have just experienced an influenza infection.

In light of the ambiguity surrounding definitions of health, it may be most helpful to define health, whether good or bad, in terms that can be evaluated and understood in light of the ability of the person being assessed to operate normally in light of the likelihood of an impending disease. Tables of "reference values" printed in clinical medicine, diagnostics, and other references of this kind provide measurements that can be found. When someone is given a health exam, there are probably going to be a number of tests involved. Some of these tests can reveal the existence of the disease in a person who appears to be healthy and is more descriptive than quantitative. These tests include electrocardiograms to look for certain types of cardiac disease, electromyograms to look for primary muscle abnormalities, liver and gallbladder function tests, and X-ray methods to look for disease or organ dysfunction.

Other tests provide numerical results that the examiner can interpret (or outcomes that can be ascribed to numerical values, like photometric color determinations). They include physical and chemical examinations, such as investigations of the cerebrospinal fluid, blood, and urine. When the test results are compared to the reference values, the doctor is given information about the patient's health and, if the values are aberrant, suggestions for how to improve the patient's health.

The interpretation of test results is significantly hampered by biological variability. Reference values for variables are often the means or adjusted means of measurements taken from large groups of people. To be meaningful, these numbers must be viewed as being within a 95 percent range, which corresponds to the so-called "ordinary range" or, with caveats, the range between normal and the upper and lower boundary limits. Therefore, the 2.5 percent below the lower limit and the 2.5 percent over the top limit of the 95 percent range are regarded as anomalous or perhaps unhealthy zones. Wide 95 percent ranges exist in several domains; for instance, blood pressure can change significantly throughout the course of the day (for instance, during physical activity, fear, or rage) while yet remaining within the normal range. Other values are referred to as physiological constants since their ranges are so constrained. When measured at the same anatomical spot, a person's body temperature, for instance, seldom varies by more than one degree (from the time they get up and when they go to bed) without being a sign of an infection or other sickness.