I know that the correct phrasing for this sentence is, but I don't know why.

Many of the volunteers had already shown some early signs of mental illness.

Since the illness is unspecified, shouldn't it be illnesses? Mental illnesses can be counted:

This person suffers from two diagnosed mental illnesses.

So why is it we say "early signs of mental illness" as opposed to "illnesses"? We're talking about an unspecified group of people which may or may not have a certain condition or several conditions.

TheFreeDictionary shares a few examples of the word "illnesses" in literature. In several of these examples, they specifically use the term "mental illnesses". Such as this one

Policy limitations for mental illnesses typically limit benefits to 24 months if the disability is "caused or contributed to" by a "mental illness.

  • 1
    Lexico considers "illness" to be a mass noun. Usually when we talk about mental issues, we use the word "disorder". Sep 26, 2019 at 20:10
  • So illnesses is never correct? Sep 26, 2019 at 20:12
  • I think it's possible to have more than one mental illness, just as it is possible to order more than one water in a restaurant.
    – Robusto
    Sep 26, 2019 at 20:53
  • Pick up a copy of DSM V: en.wikipedia.org/wiki/DSM-5 It's possible, even common, to have multiple diagnoses. Sep 26, 2019 at 20:56
  • I'm guessing arithmomanias would be.
    – Phil Sweet
    Sep 27, 2019 at 0:13

3 Answers 3


"Illness", when used that way, can refer to any kind of illness, or even multiple kinds of illness. Maybe there are some situations where "illnesses" would be correct, but for the most part you'd almost never use it.

I think the reason is that, in this case, "illness" refers the presence of negative symptoms rather than a specific diagnosis. Whether a person is diagnosed with one single disorder, like depression, or multiple separate disorders, like bipolar disorder, borderline personality disorder, and body dysmorphia, you still would describe this person as "suffering from mental illness", not "illnesses". So whether you're referring to a single person with one problem, a single person with several different problems, or multiple people with multiple different problems, you use "illness" as a singular noun.

  • books.google.com/ngrams/…
    – Hot Licks
    Sep 27, 2019 at 0:16
  • The British Psychological Society stated in its June 2011 response to DSM-5 draft versions, that it had "more concerns than plaudits".[67] ... It also expressed a major concern that "clients and the general public are negatively affected by the continued and continuous medicalisation of their natural and normal responses to their experiences... which demand helping responses, but which do not reflect illnesses so much as normal individual variation". en.wikipedia.org/wiki/DSM-5
    – Hot Licks
    Sep 27, 2019 at 0:24
  • What is the point you're tying to make in these comments? "Illnesses" has some usage, but "illness" is far more common. In the usage described in the original question, "illnesses" is very, very rare. books.google.com/ngrams/…
    – sky
    Sep 27, 2019 at 3:44
  • So schizophrenia, Alzheimer's, and autism are all the same illness??
    – Hot Licks
    Sep 27, 2019 at 12:06
  • books.google.com/ngrams/…
    – Hot Licks
    Sep 27, 2019 at 12:36

You fail to understand that a medical term may be used to refer to a category of disease or to a specific type of disease.

In "Many of the volunteers had already shown some early signs of mental illness" the term "illness" is used to refer to a category, but in "This person suffers from two diagnosed mental illnesses" the reference is to specific types.

You might say "The patient is suffering from two different types of cancer", but you would not say "The patient is suffering from two different cancer." However, "The patient is suffering from two different cancers" is valid.


illness MacMillan an uncountable noun

  • [uncountable] the state of feeling sick or of having a disease
    • General words for illnesses, diseases and medical conditions: illness, disease, sickness...

As in:

Many patients (plural) are recovering from major surgery or serious illness (uncountable noun).


Various illness spread throughout the land.

But I would be remiss if I did not mention Longmans's Dictionary lists illness as countable and uncountable.


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