In this text:
New Yorkers are contending with a bewildering and miserable mix of symptoms as Covid, flu, R.S.V. and various mystery illnesses circulate.
Why "mystery illnesses" instead of "mysterious illnesses", if "mysterious" is an adjective, which would be more suitable to go before a noun? Is there any special case for using a noun to describe another noun when there is an adjective for it?
"mystery" used in this way means "unidentified or unexplained". This is subtly different from "mysterious" which means "difficult to understand or account for; of a nature hard to reconcile with what is known".
To call them mysterious illnesses would suggest that they're puzzling and perhaps even impossible to understand. To call them mystery illnesses could mean just that we don't know where they came from.
There is some overlap between the two words, but that's as close as I can get to delineating the difference. Also, compare "mystery meat". In such a case you don't know what type of meat it is. But if it were "mysterious meat" you would say there was something unusual or strange about it.
"Mystery", in this kind of context, means unknown.
"Mysterious" means more than that... it can mean strange, uncanny, unusual, or difficult to explain.
When you think about it, there's often nothing strange or uncanny about an illness. Even most new diseases cause familiar symptoms and turn out to be new strains of something we already know about. So it could seem a little anti-science to say an illness was 'mysterious'. But saying it is a 'mystery illness' just means it is as yet unidentified.
Mystery and mysterious can both work as adjectives, and have subtle differences in meaning.
Mystery (as an adjective before a noun) often means "currently unknown" or "not yet revealed". For instance, a talk show could have a mystery guest - someone who is coming onto the show, but the audience won't know who yet.
Mysterious means illusive, strange, or hiding something. So if a talk show had a mysterious guest, this describes the guest's behaviour or personality - for instance they might be involved in the occult, or someone who is avoiding answering questions.
There’s a similar term, “mystery religion”, which originally described any one of a number of Roman cults that were centred around a “mystery”: rites and teachings which were kept secret from outsiders but which were progressively revealed as one became an initiate and rose through the ranks. Consequently, I expect that when the word “mystery” is used as an adjective in this way it means “something identifiable but whose essential nature is not presently known”. COVID, for instance, could have initially been described both as a mystery illness (“people are contracting a disease we haven’t yet identified”) and as a mysterious illness (“we don’t understand why this disease causes such odd symptoms”). It’s still somewhat mysterious, but since we know the cause it’s no longer a mystery illness.
The difference between “mystery” as an adjective and “mysterious” is somewhat allied to the masked-man fallacy.
The illness is not necessarily mysterious in itself: it might not be subtle or complex or difficult to understand. We just don’t know what it is. Its identity is mysterious, because of shortfalls in our own information.
(Also, “mystery” as an adjective is typically used in a somewhat jocular way. “Who is the mystery man?” you might teasingly ask a friend with a new and unnamed beau. “Mysterious” is said more in admiration or concern.)
