Why is a definite article used before flu?

You're right to notice that we say "the flu" and not just "flu." Here's why:

* Flu is actually short for influenza, which is a specific disease.

* The definite article "the" is used to refer to something specific or unique. Since influenza is a specific illness, we use "the" to indicate that we're talking about that particular disease, not any general illness.

It's similar to how we say "the common cold" because there's a specific condition known as the common cold.

Let me know if you'd like to explore other interesting grammar points!