Learn Italian: Siccome l'influenza è grave, il dottore deve curare mio nonno. - Since the flu is serious, the doctor must treat my grandfather.

Siccome means 'since' or 'because' and is used at the start of a sentence to introduce a cause. If you want to use perché ('because'), it usually goes in the middle of the sentence instead: Il dottore deve curare mio nonno perché l'influenza è grave.

Does influenza mean 'influence' or 'flu' here?

It means 'the flu'. Just like in English, the medical illness is called influenza. You can easily tell which meaning is intended here because of the medical context of a doctor (il dottore) treating (curare) someone.

Why does grave end in 'e' when it describes a feminine noun like l'influenza?

The adjective grave (serious or severe) belongs to a group of Italian adjectives that end in -e for both masculine and feminine singular nouns. It only changes to an -i for the plural (gravi).

Why is there no article like 'il' before mio nonno?

In Italian, you drop the definite article (il, la, ecc.) before possessives when talking about singular, close family members. This is a common rule learners forget! Because of this rule, it is always mio nonno, never il mio nonno.