Learn Italian: Ho mal di testa e la febbre. - I have a headache and a fever.

In Italian, you usually say you have 'ache of head' (mal di testa) or 'ache of stomach' (mal di pancia) without using an article before it. It acts as a single, fixed expression.

Why does febbre get the definite article la when English says 'a fever'?

Unlike mal di testa, many other common illnesses in Italian take the definite article. You say ho la febbre (literally 'I have the fever'), ho la tosse (I have a cough), and ho il raffreddore (I have a cold).

How could I use this sentence with siccome to explain why I am staying home?

Siccome (since or because) is used to set up the cause at the very beginning of a sentence. You would say: Siccome ho mal di testa e la febbre, resto a casa. (Since I have a headache and a fever, I am staying home.)

Could I use perché instead of siccome to say the same thing?

Yes, but perché normally joins two ideas in the middle of a sentence. You would need to flip the order: Resto a casa perché ho mal di testa e la febbre. (I am staying home because I have a headache and a fever.)