Disease

A disease is a condition that changes how a living thing works or looks in a bad way. It is not because of a sudden injury, like falling down. Diseases often come with clear signs and symptoms that doctors can recognize. They can be caused by things from outside the body, like tiny germs, or by problems inside the body, such as when the body’s defense system, the immune system, does not work right. This can lead to many different health problems.

In humans, disease can mean any problem that causes pain, makes it hard to do things, or even leads to death. It can also cause trouble for families and friends of the person who is sick. Diseases can affect how a person thinks and feels, too, because having a disease can change how they see the world.

There are four main types of disease: infectious diseases, which spread from person to person; deficiency diseases, caused by not getting enough nutrients; hereditary diseases, which are passed down from parents; and physiological diseases, which come from how the body works. Some of the deadliest diseases are ischemic heart disease, stroke, and COPD. In places where people live longer, mental health problems like depression and anxiety are very common causes of sickness. Studying diseases is called pathology, and it includes learning about what causes them, such as human immunodeficiency viruses that cause the syndrome of acquired immunodeficiency.

Terminology

Concepts

In many cases, words like disease, disorder, sickness, and illness are used in the same way. However, sometimes we use specific words to be more clear.

Disease

The word disease means any condition that makes the body not work right. Diseases happen when the body’s normal balance is upset. Often, diseases come from tiny germs like viruses or bacteria. But not all diseases come from germs. Some come from inside the body, like when parts of the body stop working properly.

Acquired disease

An acquired disease starts sometime after a person is born. This is different from diseases that a person is born with.

Acute disease

An acute disease is one that doesn’t last long, like a cold.

Chronic disease

A chronic disease lasts a long time, often for many years. These diseases might come and go or stay all the time.

Congenital disorder

A congenital disorder is one that a baby is born with. It might come from changes in their genes or from something the mother caught while pregnant.

Genetic disease

A genetic disease happens because of changes in the genes. These changes can be passed down from parents or happen on their own.

Hereditary disease

A hereditary disease is a genetic disease that runs in families.

Iatrogenic disease

An iatrogenic disease is one that happens because of a medical treatment.

Idiopathic disease

An idiopathic disease is one where we don’t know the cause yet.

Incurable disease

An incurable disease is one that can’t be completely fixed. But treatments can help make the person feel better.

Primary disease

A primary disease is the main reason someone is sick. Other problems might happen because of this main disease.

Secondary disease

A secondary disease is a problem that happens because of another, main disease.

Terminal disease

A terminal disease is one that will likely lead to death.

Illness

The words illness and sickness can mean the same as disease. But sometimes, illness means how a person feels when they are sick. A person might have a disease but not feel sick, or they might feel sick even if they don’t have a disease.

Disorder

A disorder is when something in the body isn’t working right. This word is often used for mental health problems, but it can be used for other problems too.

Medical condition

A medical condition is a wide word that means any health problem, like a disease or injury.

Morbidity

Morbidity means being sick or having a health problem. It can talk about how many people have a certain disease or how sick someone is.

Pathosis or pathology

Pathosis and pathology both mean disease.

Syndrome

A syndrome is when a group of signs or symptoms happen together. Sometimes we know why, and sometimes we don’t.

Predisease

Predisease is when someone is likely to get a disease soon, but they don’t have it yet.

Types by body system

Mental

Mental illness is a general word for problems with thinking, feeling, or behaving. Examples include sadness that doesn’t go away, feeling very worried, and trouble thinking clearly.

Organic

An organic disease is one that comes from a real change in the body, like after an accident or injury.

Stages

For some diseases caused by germs, there is a time between when a person gets the germ and when they start feeling sick. Some diseases can hide in the body for a long time before making someone sick again.

Acute disease

An acute disease is short, like a cold.

Chronic disease

A chronic disease lasts a long time, often many years. It might get better sometimes and worse other times.

Clinical disease

This is when a disease starts to show clear signs and symptoms.

Cure

A cure ends a disease forever. Sometimes, a disease goes away for a while but comes back later.

Flare-up

A flare-up is when a disease gets worse suddenly.

Progressive disease

A progressive disease gets worse over time.

Refractory disease

A refractory disease is hard to treat.

Subclinical disease

This is when a disease is present but there are no symptoms yet.

Terminal phase

This is the time close to the end of life for someone with a serious disease.

Recovery

Recovery means getting better after being sick. The body heals and starts working normally again.

Extent

Localized disease

A localized disease affects just one small part of the body, like a foot or an eye.

Disseminated disease

A disseminated disease has spread to many parts of the body.

Systemic disease

A systemic disease affects the whole body.

Classification

Main articles: Nosology and Medical classification

Diseases can be grouped by what causes them, how they develop, or by the signs and symptoms they cause. They can also be grouped by which body system they affect, although many diseases impact more than one part of the body.

One challenge in grouping diseases is that sometimes we don’t fully understand what causes them, so doctors may describe them by the symptoms they cause instead. The most commonly used way to group diseases comes from the World Health Organization, called the ICD, which gets updated from time to time. The latest version is ICD-11.

Causes

See also: Cause (medicine) and Transmission (medicine)

Diseases can happen because of many different reasons. They might come from tiny living things like bacteria or viruses, from our genes, or from things around us in the environment. Sometimes, a mix of these things can cause a disease.

Some diseases, like influenza, can spread from person to person. These are called infectious diseases. They can spread through things like touching contaminated surfaces, being bitten by insects or other carriers, or even through food or water. Other diseases, like most types of cancer and heart disease, are not spread between people and are called non-infectious diseases. These often come from our genes or our lifestyle choices.

Where we live and our living conditions also affect our health. Things like where we live, how much money we have, and the environment around us can all influence whether we get sick. Health groups like the Public Health Agency of Canada and the World Health Organization know that these social conditions are very important for our health.

Types of causes

Airborne

An airborne disease is any disease that spreads through the air.

Foodborne

Foodborne illness or food poisoning happens when eating food that has harmful things in it.

Infectious

Infectious diseases, also called transmissible or communicable diseases, are illnesses that come from tiny living things getting inside our bodies. These can spread between people. Examples are influenza and the common cold.

Lifestyle

A lifestyle disease is a disease that becomes more common in places where people live longer and work less. It often comes from choices like not moving much or eating unhealthy foods.

Non-communicable

A non-communicable disease is a disease that cannot spread from one person to another. Examples are heart disease and cancer.

Prevention

Main article: Preventive medicine

Many illnesses can be stopped before they start. Good habits like keeping things clean, eating healthy food, staying active, and getting vaccinations help keep us safe. Other steps, like wearing cloth face masks and following public health rules, also protect us from getting sick.

Treatments

Main article: Therapy

Medical treatments are ways to help cure or improve health problems caused by diseases. These can include medications, surgery, medical devices, and self-care. Treatments can be given by doctors in a health care system or sometimes by the patient or family at home.

Some treatments help prevent illnesses before they start, called preventive healthcare. Other treatments are used after an illness has begun to make someone feel better. Not all treatments can fully cure a disease, especially for long-lasting chronic diseases, but they can still help manage the problem. For sudden and serious health issues, called medical emergencies, quick care is needed, often in an emergency department or an urgent care center.

Epidemiology

Main article: Epidemiology

Epidemiology is the study of what causes diseases and what makes them spread. Some diseases happen more in certain places, in certain groups of people, or during specific times of the year.

Epidemiology helps us understand how to keep people healthy. It looks at how diseases affect groups of people and uses many kinds of science to find out why diseases happen. This helps us prevent diseases and improve health for everyone.

Burdens of disease

Disease burden shows how much a health problem affects people, measured by things like money lost, or how many people get sick or die.

There are ways to measure how much disease affects people. One way is to count how many years of life are lost because of a disease. Another way looks at both how many years are lost to death and how many years people spend feeling sick. These measurements help us see which diseases cause the most problems, whether they kill quickly or make people sick for a long time. In the world, heart disease and stroke cause many deaths, but conditions that make people feel very sick, like major depression, cause many years of feeling unwell.

Society and culture

How people and societies think about and react to diseases is an important area of study called medical sociology.

Some conditions might be seen as diseases in one culture or time but not in another. For example, obesity used to be linked with wealth and plenty, and this idea still exists in some parts of Africa, especially since the start of HIV/AIDS. Epilepsy is viewed as a sign of special spiritual gifts among the Hmong people.

Being sick can bring certain benefits, like getting time off work or extra care from others. When someone is sick, they take on a special role in society called the sick role. People with serious diseases, like cancer, might be respected more in their communities for handling their illness in a way that others see as proper. In return for these benefits, the person is expected to try to get better. This is different from pregnancy, which is not seen as a sickness even though it might need medical care.

Most religions allow exceptions for people who are sick. For instance, someone who might be in danger if they fast during special religious times like Yom Kippur or Ramadan does not have to fast. Being sick is also a good reason to skip certain social duties. In the United States, feeling ill is one of the few reasons someone can politely decline an invitation to important places like the White House.

Labeling a condition as a disease instead of just a normal human variation can change many things about a person's life and the responsibilities of governments, companies, and institutions. Recognizing conditions like repetitive stress injury (RSI) and post-traumatic stress disorder (PTSD) has had many effects on how society supports individuals. Seeing aging as a disease could change a lot, though this idea is not commonly used yet.

People who had leprosy in the past were often avoided because of their disease, and the word “leper” still carries a feeling of social stigma. Fear of disease can affect whole societies, even if not all diseases are feared the same way.

A person’s wealth and social status can affect their health. Diseases of poverty are linked with being poor, while diseases of affluence are linked with being wealthy. Which diseases belong to which group can change depending on where and when you live. Some diseases, like diabetes, can be connected to both poverty and wealth, but for different reasons. The term lifestyle diseases refers to diseases that happen more often in older people.

People often tell stories about their health problems to explain what they’ve gone through. They use comparisons, like thinking of disease as something to fight against, similar to a war. Sometimes they describe being sick as a journey, like traveling toward getting better. Different diseases can bring different kinds of feelings and ideas — for example, some people think of addiction as being trapped, or they might see losing hair from chemotherapy as a symbol of all the losses from illness.

Some diseases are used to represent bigger problems in society. For example, people might talk about “cancer” to describe something harmful spreading in communities, like unfairness or poverty. In the past, diseases like tuberculosis were seen in literature as a way people rose above everyday life.