Disease

A disease is an abnormal condition that affects how an organism functions or how its body is structured. It is distinct from an external injury, such as a broken bone or a cut. While many people use the terms disease, illness, and sickness interchangeably, they can have different meanings in medicine. A disease often refers to an objective medical condition that disrupts homeostasis, which is the body's internal balance. In contrast, illness often describes a person's subjective, personal experience of being unwell.

Diseases arise through several different biological mechanisms. They can be caused by external factors known as pathogens. These include bacteria, viruses, fungi, protozoa, multicellular organisms, and even prions, which are aberrant proteins. Other diseases result from internal dysfunctions. For instance, the immune system may malfunction, leading to autoimmune disorders where the body attacks itself. Some conditions are also iatrogenic, meaning they are caused by medical interventions or treatments. If the cause of a condition is completely unknown, scientists call it an idiopathic disease.

Medical professionals classify diseases into several specific categories based on their timing and origin. An acute disease is short-term in nature. Conversely, a chronic disease persists for a long time, often lasting at least six months or even a lifetime. We can also categorize them by when they begin. A congenital disease is present at birth, sometimes due to genetic factors or infections passed from a mother. An acquired disease is any condition that develops at some point during a person's life.

There are also distinctions between primary and secondary diseases. A primary disease is the root cause of an illness. A secondary disease, or sequela, is a complication that results from that primary cause. For example, a primary viral infection might weaken the immune system. This weakness could then lead to a secondary bacterial infection. This relationship helps doctors determine the correct treatment, such as deciding if antibiotics are necessary for a patient's symptoms.

Scientists study these conditions through a field called pathology. This is the study of disease itself. A major part of pathology is etiology, which is the study of the causes of disease. By studying etiology, researchers can identify specific causal agents, such as the human immunodeficiency viruses (HIVs) that lead to AIDS. Understanding these causes is essential for developing treatments and preventing the spread of infections.

In human populations, the impact of disease is measured in different ways. Morbidity refers to a diseased state, disability, or poor health. It can also describe the frequency or prevalence of a condition in a population. This is different from mortality, which is the rate of death. In humans, some diseases are particularly deadly. Ischemic heart disease, which involves the obstruction of blood flow, is a leading cause of death. This is followed by stroke and chronic obstructive pulmonary disease (COPD).

Disease also has significant social and mental dimensions. In humans, the term is often used broadly to include conditions that cause pain, distress, or social problems. Diseases can affect a person's mental state, altering their perspective on life. In many developed countries, neuropsychiatric conditions like depression and anxiety are among the most common causes of sickness. Because these conditions involve complex interactions between biology and psychology, they are often referred to as mental disorders.