Disease

• 1. Introduction

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  A disease is an abnormal condition that harms the structure or function of all or part of an organism, and it is not the immediate result of an external injury. That definition sounds clinical, but it sits at the center of how humans understand suffering itself. The deadliest of these conditions in humans is ischemic heart disease, a blockage of blood flow, followed by stroke and a lung disease called COPD. Yet in developed countries, the conditions that cause the most sickness overall are neuropsychiatric, things like depression and anxiety. So what counts as a disease, and what does not? Why do a cold and a bacterial infection demand different treatment? And why do societies so often turn an illness into a metaphor for everything they fear? The answers reveal that disease is as much a human idea as a biological fact.

• 2. A Tangle Of Names

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  Disorder, morbidity, sickness, illness, pathosis. Physicians often use these words interchangeably, yet each carries a shade of meaning that matters in the right moment. The term disorder is considered more value-neutral and less stigmatizing than disease or illness, which is why mental health professionals favor it. It acknowledges the complex interaction of biological, social, and psychological factors in psychiatric conditions.

  Illness, by contrast, sometimes refers specifically to the patient's personal, subjective experience of their disease. This opens a strange gap. A person can have a disease without being ill, carrying a subclinical infection that produces no symptoms. A person can also be ill without being diseased, as when someone feels unwell from embarrassment and reads those feelings as sickness rather than normal emotion. Symptoms of illness are often not the direct result of infection at all. They are a collection of evolved responses, a sickness behavior that includes lethargy, loss of appetite, sleepiness, and an inability to concentrate, all working to clear infection and promote recovery.

  Morbidity describes a diseased state or poor health from any cause. In epidemiology and actuarial science, the term takes on a numerical life. A morbidity rate can mean the percentage of people who experience a condition within a timeframe, such as 20% of people getting influenza in a year. Health insurers, life insurers, and long-term care insurers use these rates to set the premiums they charge customers.

• 3. Sorting The Causes

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  Microorganisms, genetics, the environment, or some combination of them can all push a body into a diseased state. The pathogens that cause contagious illnesses include bacteria, viruses, protozoa, and fungi. They travel by hand-to-mouth contact with infectious material on surfaces, by the bites of insects, through contaminated water or food, and through sexual transmission.

  Non-infectious diseases form the other great category, including most forms of cancer, heart disease, and mental disorders. Many of these have a partly or completely genetic basis and can be passed from one generation to the next. A genetic disorder is caused by one or more mutations, often inherited, though some arise randomly and de novo. An acquired disease, despite how the word sounds, simply means one that began at some point after birth rather than being present from the start, which would make it congenital.

  Social conditions shape health as powerfully as any germ. The World Health Organization and the Public Health Agency of Canada both recognize that the social, economic, political, and environmental circumstances people live in greatly influence their well-being. Climate change can worsen the transmission and burden of some infectious diseases. When a cause is poorly understood, societies tend to mythologize the disease. Until the bacterial cause of tuberculosis was discovered in 1882, experts blamed it on heredity, a sedentary lifestyle, depressed mood, and overindulgence in sex, rich food, or alcohol, all of which were considered social ills at the time.

• 4. Primary And Secondary

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  A common cold is a primary disease, but the rhinitis that follows it can be a secondary one, a sequela or complication of the original cause. This distinction is not academic. A doctor must determine whether a cold or a bacterial infection is driving a patient's rhinitis before deciding whether to prescribe antibiotics.

  Secondary disease often begins where a primary illness leaves the body vulnerable. A primary viral infection that weakens the immune system can open the door to a secondary bacterial infection. A burn that creates an open wound can give bacteria an entry point, leading to infection that the burn itself did not cause. The chain matters because treating the complication without addressing its root rarely resolves anything.

  Some diseases resist any treatment at all. A refractory disease fights off treatment more than is normal for that particular condition. An idiopathic disease has an unknown cause, though that status often fades as science advances. When germs were discovered, infection gained an explanation, even before particular germs were linked to particular diseases. Autoimmunity is now known to cause some forms of diabetes mellitus type 1, even though the precise molecular pathways are not yet understood.

• 5. Stages And Hiding Places

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  Varicella zoster virus first causes chickenpox in its acute phase. After a person recovers, the virus does not always leave. It can remain dormant in nerve cells for many years, then reawaken to cause herpes zoster, the painful condition known as shingles. This is viral latency, a phase in which the virus hides in the body in an inactive state.

  The incubation period is the stretch between infection and the first appearance of symptoms. The latency period marks the time between infection and the moment a disease can spread to another person, which may come before, after, or alongside any symptoms. Some diseases pass through a subclinical or silent stage before symptoms are ever noticed.

  Clinical disease is the stage that produces the characteristic signs and symptoms physicians recognize. AIDS, for example, is the clinical disease stage of HIV infection. A cure ends a medical condition, while remission refers to the disappearance of symptoms, possibly only temporarily. For incurable diseases, complete remission is the best possible outcome. A progressive disease tends to worsen until death, serious debility, or organ failure, while a stable or static disease exists without getting better or worse.

• 6. Counting What Is Lost

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  In 2004, the World Health Organization calculated that 932 million years of potential life were lost to premature death. This figure comes from a measure called years of potential life lost, or YPLL. If a person dies at 65 from a disease and would likely have lived to 80, that disease has cost 15 years of potential life.

  YPLL has a blind spot. It treats a person who dies suddenly and a person who dies at the same age after decades of illness as equivalent, because it does not account for disability before death. The quality-adjusted life year and the disability-adjusted life year fill that gap by adding part of the years lost to being sick. A disease with high morbidity but low mortality carries a high DALY and a low YPLL. In 2004, the World Health Organization calculated that 1.5 billion disability-adjusted life years were lost to disease and injury.

  The pattern of loss shifts by region. Worldwide, infectious and parasitic diseases account for 37% of all YPLLs lost, with lower respiratory tract infections, diarrhea, AIDS, tuberculosis, and malaria leading the way. In Europe, cardiovascular diseases account for 35% of YPLLs lost. In the US and Canada, neuropsychiatric conditions like depression account for 28% of all DALYs lost, far more than their share of life lost outright.

• 7. The Meanings We Assign

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  Epilepsy is considered a sign of spiritual gifts among the Hmong people. That single fact captures how completely a society decides what a disease means. Obesity was once associated with prosperity and abundance, a perception that persists in many African regions, especially since the beginning of the HIV/AIDS era. A condition counted as disease in one culture or era may not be in another.

  Sickness grants a social role with real benefits. The sick person earns illness benefits, work avoidance, and care from others, and in return is obligated to seek treatment and work toward recovery. Most religions excuse the sick from duties. One whose life would be endangered by fasting on Yom Kippur or during Ramadan is exempted, or even forbidden, from taking part. In the United States, ill health is the only socially acceptable reason to refuse an invitation to the White House.

  Language turns disease into something larger than itself. The most popular metaphors are military. Disease is an enemy to be fought and routed, the patient a warrior rather than a passive victim, as in the war on cancer. This empowers some patients and leaves others feeling like failures. British healthcare professionals lean more often on the metaphor of illness as a journey, a road to recovery or a path to travel. Tuberculosis shows how these meanings invert over time. In the 19th century, authors used it as a symbol of transcendence, its sufferers portrayed as ephemeral objects of spiritual or artistic achievement. In the 20th century, once its cause was understood, the same disease became the emblem of poverty and squalor.