Illness is a deviation from normal health, either physically or mentally, based on an individual's perception. It may or may not be related to a disease condition. Risk factors for illness are considered modifiable or non-modifiable. In the majority of cases, we find that modifiable factors give rise to non-modifiable risk. Let us take the example of cigarette smoking. Smoking is a modifiable risk factor, and the repeated use of cigarettes may lead to lung cancer—a non-modifiable risk. Many risk factors can increase the possibility of illness. Age is one such risk factor, especially in children of school-going age and in older adults. Genetic factors are also considered to be risk factors for illnesses such as cancer and diabetes. Physiologic factors like obesity increase the possibility of heart disease. Lifestyle habits such as smoking or poor nutrition may lead to many health problems. Other factors, such as working environments with exposure to hazardous materials or stress can also lead to illness.