Translation guide
The English word 'flu' refers to influenza, a common viral illness. In Japanese, the most common and natural way to say this is インフルエンザ (influenza), often shortened to インフル in casual speech. There is also a traditional term 流行性感冒, but it is rarely used in everyday conversation.
I have the flu.
Referring to the viral respiratory illness commonly known as the flu.
The standard Japanese word for influenza. Used in all contexts, from casual to formal.
The flu is going around this year.
I caught the flu.
A common casual abbreviation of インフルエンザ. Used in everyday conversation, but avoid in formal writing.
I've been in bed with the flu since yesterday.
The formal medical term for influenza. Rarely used in daily life; mostly seen in medical contexts or older texts.
Influenza is caused by the influenza virus.
インフル is perfectly fine in casual conversation with friends and family. In more formal situations or writing, use the full インフルエンザ. For example, when calling in sick to work, it's safer to say インフルエンザです.
Sorry, I'll be absent due to the flu.