Major Depressive Disorder is a crippling condition.
In the severest form, as well as a greater intensity of the symptoms mentioned in the classification system called the DSM (see Is The Rate of Depression Actually Increasing or Not?), there can also be what are called psychotic symptoms in the form of delusions and hallucinations.
A delusion is a firmly held fixed belief that is out of keeping with the person's social and cultural background. It is held with strong conviction despite evidence to the contrary. Types of delusions that are present can relate to feelings of worthlessness, guilt or persecution. So called delusions of reference are that insignificant remarks, events, or objects in one's environment have personal meaning or significance.
A hallucination is a perception in the absence of an external stimulus that at the same time has the quality of a real perception. Hallucinations can manifest in many forms. Examples include voices when no one is present or seeing things that are not actually there.
It is important to mention that psychotic symptoms are not always a feature of mental illness and can be present at the time of waking up or going to sleep, as well as when very tired.
It can be helpful to distinguish between 4 forms of MDD:
1. The symptoms are always present and never go away.
Under the stresses of life the constant low-level depression becomes worse. This may be for a few weeks or a few months. You can compare it to a being like a constant dull headache that suddenly becomes sharp now and again. A sudden setback or disappointment; a difficult meeting or confrontation with someone. Such things happen to all of us, but most of us don't develop clinical depression as a result. The old name for this was "neurotic depression".
Usually there are symptoms of anxiety that go along with the general pessimistic and fearful attitude to life. On reflection I think this is the form I am most prone to (see video Just As I Am).
When my children were younger, I was reminded of this when we had a fun exercise of giving each other nicknames. It was a salutatory wake up call when my children decided that my nick name should be 'Puddleglum'! In case you don't know Puddleglum is a fictional character from C.S. Lewis' children's novel, "The Silver Chair". He is caricature of pessimissim and a bastion of gloomy fortitude. In the past the word 'neurotic' was used to describe such people and more recent terms include 'dysthymia' and 'generalised anxiety disorder'.
2. The symptoms of depression come and go with periods when they are completely or nearly completely healthy.
When they are unwell they can be deeply sick, seriously low in mood, non-functional and even suicidal. So called episodes of depression can happen alone (s0 called unipolar depression) or with periods of high mood called mania (so called bipolar depressive disorder).
3. With some people they are generally healthy with no symptoms of depression. They have a severe depressive episode and then then they never have another one.
It is estimated that this accounts for about 1/3 of people who develop MDD.
4. A medical condition causes the MDD.
Such people are generally very healthy. But then they develop a physical illness such as hypothyroidism, a stoke, or a heart attack or cancer. If the medical illness is cured then the depression goes away never to return.
For an example of the devastating personal impact of MDD see Rick Warren and the Anguish of Major Depressive Disorder.
These 4 forms of MDD also reveal a spectrum of symptoms that range from neurotic low-level symptoms of anxiety and depression that can persist over many years to a single episode (with or without a medical cause) to recurrent depressive episodes.
It is MDD that is the illness part of what gets called depression. However, it is important to reflect that most of what gets called depression is not actually in the strict sense actual illness (see Why has there been a 400% increase in the prescribing of antidepressants?)
What questions do you have about MDD as an illness?