Arabic Dua
بِسْمِ اللَّهِ الْكَبِيرِ، نَعُوذُ بِاللَّهِ الْعَظِيمِ مِنْ شَرِّ كُلِّ عِرْقٍ نَعَّارٍ، وَمِنْ شَرِّ حَرِّ النَّارِ

Transliteration
Bismillahil kabeer, na'oozu billahil azeem min sharri kulli 'irqin na'aar, wa min sharri harrin naar.

Translation
"In the name of Allah, the Great. We seek refuge with Allah, the Mighty, from the evil of every inflamed vein and from the evil of the heat of the fire."

Explanation
When to read:

This dua is for when someone has fever.

Prophet Muhammad (ﷺ) used it to seek relief from fever and inflammation.

Meaning:

You start by taking Allah’s name, recognising His greatness.

You ask protection from the evil of all inflamed veins (the cause of fever and inflammation).

And from the harmful heat like the heat of fire (because fever is like internal heat).

How to use it:

Recite it with conviction.

You can blow lightly on the patient after reciting.

Also, combine it with medical treatment as taught in Islam (Prophet ﷺ said to seek treatment along with dua).