In everyday English, you can place the article “the” with some traditional folk names of diseases, such as the flu, the measles, or the chicken pox.
My child has the flu.
In formal contexts or professional language, we often drop the article.
You can usually treat flu yourself by resting and drinking plenty of water.
Since flu is an uncountable noun, you cannot use the article “a” with it.
1. Flu or The Flu
Generally, we do not use articles with names of diseases or illnesses (cancer, diabetes, etc.).
If you have diabetes, you need to manage your blood sugar levels.
However, in informal or natural conversations, you can add the article “the” with some traditional names of diseases, including “the flu”, “the chicken pox”, or “the measles”.
The whole family has the flu.
The flu can be severe and lead to serious complications.
In professional or more formal contexts, we typically omit the article.
People who have flu often experience headaches and fatigue.
Most people with flu do not need antiviral drugs.
You cannot use the indefinite article “a” with this word because flu is an uncountable noun.
My sister has the flu.
My sister has a flu.
The word flu can also serve as an adjective. In this situation, you can use the article a/an according to the usual rules.
The protection provided by a flu vaccine varies from season to season.
Coconut oil is anti-viral and antibacterial, but it's not the best choice to use as a flu remedy.
Follow the same convention with other related expressions, such as “treat flu”, “spread flu”, “get the flu”, etc.
Typically, you'll need nothing more than rest and stay hydrated to treat the flu. (or treat flu)
Wash your hands to keep from spreading flu to others. (or spreading the flu)
2. The Terms Influenza and Grippe
Flu is short for influenza. We normally use the word influenza in formal and professional situations.
Influenza is a respiratory infection caused by viruses.
Influenza can cause mild to severe illness.
When using influenza in generic terms, avoid the article “the”.
Having influenza is entirely different from having a cold.
Your doctor may suggest that you be tested for influenza.
Grippe is an old-fashioned term for the flu.
My grandfather would say that I have the grippe.