A disease is a particular unusual condition that affects part or all of a being not caused by external force (‘injury’) and that consists of a disorder of a structure, typically its serving as an evolutionary disadvantage. Disease is also frequently taken to a medical condition to relate to a specific signs. It can also be affected through external factors such as Germ or by internal dysfunctions, mostly the protected system, for example an immunodeficiency, or through hypersensitivity or allergies, autoimmunity.

When affected by germ the term disease is often falsely used even in the scientific works in place of its causal agent, the germ. This philological habit can cause confusion in the communication of the cause-effect principle in epidemiology, and as such it should be strongly discouraged.

In humans, disease is often used mostly to refer to any condition that causes pain, dysfunction, suffering, communal problems, or death to the person who is suffering or similar problems for those in touching with the person. In this sense, it sometimes contains injuries, disabilities, syndromes, infections, isolated signs, different behaviors, and atypical variations of structure and function, whereas in further circumstances and for other purposes these may be considered different types. Diseases can also affect people not only physically, but also emotionally, as constricting and living with a disease can change the affected person's perspective on life.

Death because of a disease is referring as death by natural reasons. There are four core types of disease: communicable diseases, deficiency diseases, genetic diseases (both genetic and non-genetic), and physiological diseases. Diseases also categorized as communicable and non-communicable. The lethal diseases in humans are coronary blood vessel disease, followed by cerebrovascular disease and lower lung infections.