A disease is a pathological of a part, organ or system of an organism resulting from various causes such as infection, genetic defect or environmental stress and characterised by an identifiable groups of signs or symptoms. It is an impairment of the normal state of the living organism or one of its parts that interrupts or modifies the performance of the vital functions, is typically manifested by distinguishing signs and symptoms, and is a response to environmental factors (as malnutrition, industrial hazards, or climate), to specific infective agents (as worms, bacteria, or viruses), to inherent defects of the organism (as genetic anomalies), or to combinations of these factors. It is a condition or tendency regarded as abnormal and harmful or any impairment of normal physiological function affecting all or part of an organism.

Disease, sometimes, includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviours and atypical variations of structure and functions.

Disease can be classified as:

Pathogenic Disease: Diseases that are caused by disease causing agents or by pathogen which could be a biological agent, or germ, that causes disease to its host by interrupting normal body processes. Examples include Cellulitis (localized or diffuse inflammation of connective tissue with severe inflammation of dermal and subcutaneous layers of the skin), Trichinellosis (a parasitic disease caused by eating raw or undercooked pork or wild gameinfected with the larvae of a species of roundworm Trichinella spiralis, commonly called the trichina worm), Lyme Disease (bacterial infection spread through the bite of the blacklegged tick), etc.

Deficiency Disease: Diseases that are caused by the lack of some particular nutrient in a person's diet which could be a nutritional or a dietary deficiency. Examples includes Scurvy (condition where an individual has a vitamin C (ascorbic acid) deficiency), Menopause (consequences of oestrogen deficiency after menopause on bone health), Goitre (swelling of the thyroid gland, which can lead to a swelling of the neck or larynx (voice box)), etc.

Hereditary Disease: A genetic or a hereditary disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Genetic disease may or may not be a hereditary disorder. It can be classified as a Single gene disorder, Multifactorial and polygenic. Examples include Tay–Sachs disease (autosomal recessive genetic disorder. that causes a progressive deterioration of mental and physical abilities that commences around six months of age and usually results in death by the age of four), Cri du chat (chromosomal condition that results when a piece of chromosome 5 is missing),  Canavan disease (inherited condition that affects the breakdown and use (metabolism) of aspartic acid), etc.

Physiological Disease: Disease in which the organs or the systems in the body malfunction causing illnesses. Examples includes Hemochromatosis (inherited disease in which too much iron builds up in your body), Anaemia (decrease in number of red blood cells (RBCs) or less than the normal quantity of haemoglobin in the blood), etc.

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