Could you please check

She has had a fever for two days
or
Her fever started two days ago.

Her fever persists.
or
She still has a fever today.

Thank you.

The sentences are incorrect. Unlike with drugs, you cannot be "on fever".

I would say:
She has had (a) fever for two days now. She is still feverish today.

I have parenthesized the indefinite article before "fever" because it is possible to treat "fever" as an uncountable noun and therefore omit the article. E.g.:
She has had fever for two days now. She still ha

Actually, further to my earlier response: for the above context, it is best to use the indefinite article before "fever". As in, "she has had a fever"…

The symptom of elevated temperature usually takes an article: "a fever of 102 degrees." But consider the lyrics to the song Fever, popularized by singer Peggy Lee:

<quote>
You give me fever
When you kiss me
Fever when you hold me tight
Fever! in the morning
Fever all through the night
</quote>

AnonymousShe has been on fever since two days ago.Her fever is carrying on today.

She's had a fever for two days and she still has it (today).

CJ