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Disease vs. Illness

Although the distinction between these two terms has been discussed on this site before, the importance of this concept justifies a fully dedicated article. Some may claim this is “just” a discussion of semantics – but it’s not.

It’s about clarity.

Understanding this particular distinction can help you make sense of many confusing things about being sick or hurt.

For example: why is healing and recovery different? Why can things hurt when there seems to be no real damage? Why don’t doctors believe in [insert fringe therapy here], yet it seemed to make you or someone you know better?

These are very good (and important) questions.

Besides helping to answer such questions, the distinction between disease and illness is one of those fundamental ideas that can help tremendously in understanding what actually works in health care (and just as importantly – what doesn’t). Knowing what doesn’t work could save your life… or at least save you money!

So let’s take a minute to understand this basic idea, and why it’s so important.


“Disease, then, is something an organ has; illness is something a man has.” – Eric J. Cassell, 1978 1

These two words are often used interchangeably. However, it would be helpful for everyone: researchers/ scientists, doctors/ healthcare professionals, and patients/ clients – if the two terms were distinguished more clearly.  2, 3, 4, 5

Without further delay, let’s define them:

Disease – best refers to an abnormal condition affecting an organism. This abnormal condition could be due to infection, degeneration of tissue, injury/trauma, toxic exposure, development of cancer, etc. This is what needs to be ‘cured’, especially if it’s life-threatening.

Illness – best refers to the feelings that might come with having a disease. Feelings like pain, fatigue, weakness, discomfort, distress, confusion, dysfunction, etc. – the reasons people seek healthcare – and usually the way people measure their success with treatment.

It’s very important to understand that feelings of illness can be vastly affected by many non-disease factors, such as expectations, beliefs, fears, feelings/moods, and culture. Being ill is a very personal experience, and can vary tremendously and be affected by very different things between people with the same ‘disease’.

The relationship between disease and illness

Of course, these two things are not mutually exclusive… let’s not create a false dichotomy here. Disease and illness are clearly related, and usually occur together.

‘Disease’ usually causes ‘Illness’ – Most of the time, a disease will be accompanied by illness. And of course, a decrease in feelings of illness can indicate the passing of a disease. 4 However, it’s worth noting that you can have one without the other.

‘Disease’ without ‘Illness’ – It’s not uncommon for a person to have a disease but have no experience of illness. People can have something as simple as high blood pressure, or as serious as a cancer, and not even know it. 4

‘Illness’ without ‘Disease’ – This is a hot topic of debate. ‘Somatoform disorders’ are those that cause physical symptoms, but no organic cause that can be detected through medical examination. Examples include ‘hypochondria’ – thinking you have a disease when you don’t, or ‘conversion disorder’ – when you start showing neurological symptoms like numbness, paralysis, or seizures, but with no neurological cause. And no, these people are not “faking it”. The debate is that these people are often considered to have a psychological disease, which may be neuro-biological in origin (and we just don’t fully understand it yet) – for instance, conversion disorder often occurs after an emotionally traumatic experience. 6

‘Illness’ can affect ‘Disease’ – It’s no surprise that disease is usually the cause of illness. But what’s really interesting is how an illness can affect a disease process, and perhaps even start it. For example, if you begin to develop a stomach ulcer because of the H. Pylori bacterium, the pain may cause you a lot of stress. But stress has been shown to cause an increase in stomach acid, and that bacteria thrives in an acidic environment! 7 Therefore, there ends up being a cycle of a disease causing an illness, the illness makes the disease worse, which makes the illness worse, and so on.

So why is this important?

  • a treatment might seem to work, but doesn’t – which could be dangerous.

The most obvious problem is that someone can feel much better after a given treatment – but still have a dangerous underlying disease. Feeling better could be due to so many other variables (which will be discussed in another article) that it can sometimes be a very poor indicator of the status of a disease. It’s an awful thought, but a person could feel great after receiving a treatment, while the disease remains in their body.

Let’s use the stomach ulcer as an example: if you give someone a placebo (sugar pill, but they think it’s a real medication), they may feel much better, and the ulcer may even heal a bit. But the bacterial infection remains, and after some time, the pain returns!

  • simply treating a disease may be insufficient.

Someone can still have feelings of illness even after a disease is ‘over’. For example, after a bad injury has healed, a person may appear fine with no signs of a serious medical issue – however, that person may still develop chronic pain. Or, after a long hospital stay fighting a serious disease, a person can be left with disability, depression, lost jobs / financial stress, and strained personal relationships.

Clearly, it’s important that healthcare treats both disease and illness.


The differences, as well as the relationships, between a disease and an illness are very interesting. When people learn the difference between the two, a lot of strange things start making a lot more sense. For examples, check out two other articles on this site: ‘Healing vs Recovery’, and ‘Damage does not ’cause’ pain’.

This is important to teach people. Learning this may relieve stress and anxiety about their feelings of illness. Furthermore, they deserve to know these things so they understand what they are looking for in healthcare.

As you can see, a treatment or therapy can seem to ‘work’, when in fact it does not. Of course, it may work to recover from feelings of illness, which is tremendously important (and usually why people come to get help in the first place!). But if a treatment claims to ‘cure a disease’ – then we better be darn sure it does.

Anecdotes and testimonials (“well it worked for me, so I know it works!”) cannot serve as a reliable basis to determine the efficacy of a given treatment or therapy on any disease or injury. This is why ‘evidence based practice’ is so important. Treatments must be tested in carefully controlled experiments to see if they truly work – and make sure it isn’t something else that seemed to cause an improvement.

Hopefully you can also see the problem with ‘placebo’ – giving someone a treatment just to make them feel better, when in fact the disease process has been unaffected. It could be dangerous, and end up doing more harm than good. Knowingly giving someone a ‘placebo’ is another hot topic of debate – involving fantastic discussions of the ethics of health care.

Interesting stuff!

Hope you learned something!

Good luck!

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    8 Responses to Disease vs. Illness

    1. Simon says:

      Juts made by evening. I was writing a research report and wondered whether to use both words on one of them-for which I did not have a suitable choice.

    2. Brid says:

      Couldnt you say that high blood pressure is the illness and the disease is what is causing these symptoms eg kidney or atherosclerosis/high cholesterol ot other cause

      • Steph says:

        There are two types of high blood pressure (HBP), primary and secondary.

        Secondary HBP is what you are referring to where the HBP is a result of an underlying disease such as renal vascular disease but, it may also be a result of naturally occurring events such as pregnancy. Basically, with secondary HBP, there is a clear cause of HBP as a symptom.

        Primary HBP however does not have this clear cause and and is therefore idiopathic. Primary HBP is the most common type of blood pressure and would therefore qualify as a disease that may cause illness (say in the event that the person has a stroke or a heart attack as a result) or may not cause illness (the person is unaware or it is controlled effectively with medication).

        Another example of a disease that may or may not cause illness would be Osteoporosis and in fact, the most common way of diagnosing Osteoporosis is after someone has had a fall and broken a bone when the amount of force would normally be insufficient to bread said bone. The person would not be aware of their condition until post fall and therefore may not experience symptoms of illness until they have been diagnosed.

        So while HBP may cause illness and be a disease without known cause (idiopathic), it may also be a result of an underlying disease (eg: renal vascular disease) or of a normal human condition (eg: pregnancy)

    3. maslah dhoore says:

      what is the difference between the sickness ,illness and disease ?

    4. Michelle Cannon says:

      So what is mental illness then? It makes me CRINGE when people refer to bipolar disorder or schizophrenia as diseases. Small pox and cancer are diseases. Colds and flu are not. Mental illness is not.

    5. Basiru mainasara says:

      are the causes of illness and disease thesame?

    6. Tolu says:

      What is the similarities and differences between sickness and illness?

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