Our cumulative knowledge about diseases shows that they occur according to a definite time scheme that may provide clues for their approaches and requisites. A certain disease known to affect an adult person may be inapplicable to mention for a child, for example. This is why noting the age of a patient is relevant to the possible diseases thought of (so-called differential diagnosis). For example, G6PD deficiency is a disease due to deficiency of a protein enzyme needed for replenishment of important detoxifying compound in the body, reduced glutathione. The disease is usually evident in young children when suffering an infection or on taking certain chemical drugs. Schizophrenia, on the other hand, is a mental disease with an onset typically in young adulthood. In this viewpoint, the life stages of a human; newborn, child, adolescent, adult and elderly, should fit for possible disease events besides their relevance to specific physical, mental and emotional characteristics.
Diseases may be classified according to their causes as follows:
1- Nutritional: caused by short of supply of food and other nutrients, e.g. vitamin deficiency.
2- Genetic: caused by certain gene defect or variation, e.g. sickle cell anemia.
3- Infectious: caused by certain pathogen as viruses, bacteria and fungi.
4- Multi-factorial: caused by different convergent mechanisms, e.g. diabetes mellitus and hypertension.
5- Traumatic: physical and chemical insults can cause inflammatory body reactions.
Foot-note: a disease may be regarded as an age- and constitution-reasoned extraordinary experience or complaint about some physical, mental or emotional state of an individual that maybe stated either by the affected person or by some people in his/her vicinity or by both.