I have a fever

[Will17]

Hello!

Is "a" necessary here, please?:

"I have a fever".

Thank you

Will

[AlexAD]

Not a teacher, nor a native.

The OALD says that 'fever' can be both countable and uncountable in the meaning of a medical condition in which a person has a temperature that is higer than normal.
But there is no example where it is uncountable.
The collocations section gives a few more examples:

to have a high temperature (BrE)
to have a fever (AmE)

So I guess the indefinite article can't be leave out here.

Let's see what teachers say.

[emsr2d2]

The indefinite article is necessary with "fever". The use (or non-use) of the article with medical conditions is one of those things that just has to be learnt, as you can see from the following list:

I have a fever.
I have a cold.
I have [the] flu.
I have [the] mumps.
I have German measles.
I have chicken pox.
I have tonsilitis.
I have laryngitis.
I have a headache.
I have earache.
I have backache.

[BobSmith]

The indefinite article is necessary with "fever". The use (or non-use) of the article with medical conditions is one of those things that just has to be learnt, as you can see from the following list:

I have a fever.
I have a cold.
I have [the] flu. AmE: I have the flu.
I have [the] mumps.
I have German measles. AmE: Also, I have [the] measles. (to my best recollection)
I have chicken pox. AmE: Same, but chickenpox is one word.
I have tonsilitis.
I have laryngitis.
I have a headache.
I have earache. AmE: I have an earache.
I have backache. AmE: I have a backache.

[not a teacher]

I also think the article is dropped when used as a symptom, such as, "The patient may present with mumps, headache, fever, and flu-like symptoms."