Physical fitness refers to good body health. It is dependent on genetic determinators and also on social, economic and ecological factors. That means, one's genes are partly responsible for one's physical health, but also other circumstances: where you live, how clean or polluted your water and the air around you is and also how good your social and medical system is. It is also the result of regular exercise, proper diet and nutrition, and proper rest for physical recovery. A person who is physically fit will be able to walk or run without getting breathless and they will be able to carry out the activities of everyday living and not need help. How much each person can do will depend on their age and whether they are a man or woman. A physically fit person usually has a normal weight for their height. The relation between their height and weight is called their Body Mass Index. A taller person can be heavier and still be fit. If a person is too heavy or too thin for their height it may affect their health. Better health is central to human happiness and well-being. It also makes an important contribution to economic progress, as healthy populations live longer, are more productive, and save more. Many factors influence health status and a country's ability to provide quality health services for its people.