If you want to say an illness in French : le mal => ache so le mal + de + a part of the body = illness or pain of that part of the body
here are some examples :
Le mal de ventre => stomachache
Le mal de tête => headache
Le mal de dent => toothache
Le mal de arrière => backache
But for cold, flu etc... That they don't depend on body parts they are different : le rhume => the cold la fièvre => the fever
But you can also say:
"J'ai mal (au/ a la) body part."
Au is for masculine nouns and a la is for feminine nouns.
J'ai mal a la tete. I have a headache.
J'ai mal a l'estomac. I have a stomachache.
J'ai mal au bras. I have an arm ache.
Use aux for plural.
J'ai mal aux dents.
To say for other people, conjugate "avoir" for the other pronouns:
Tu as mal
Elle/Il/On a mal
Elles/Iles ont mal
Nous avons mal
Vous avez mal
Also you need to use the right verb for the sicknesses.
In French, you have hot, not are hot. J'ai chaud, not Je suis chaud, for example.
The word "backache" in French is "mal de dos".
I suppose that in German one can build constructs like (body part that hurts) + (how does it hurt) + (how much does it hurt) + (how long does it hurt) + "merzen" into single word, and this will be ok to say to a doctor. Maybe just in some other order.
Wow! All in one word? How would you say a mild, burning earache that lasts several days?