Here's why:
* Common noun: "Flu" is a common noun, referring to the general concept of influenza. Common nouns are not capitalized unless they are the first word of a sentence.
* Proper nouns: Proper nouns are specific names of people, places, or things and are always capitalized. For example, you would capitalize "Influenza A" as it is a specific type of influenza.
* First word of a sentence: As with any other word, "flu" would be capitalized if it is the first word of a sentence.
Examples:
* Incorrect: The Flu is a contagious respiratory illness.
* Correct: The flu is a contagious respiratory illness.
* Correct: Influenza A is a common type of flu.
* Correct: Flu can be prevented with a vaccination.
* Correct: Flu. It's important to get vaccinated.
Let me know if you have any more questions!