Influenza.
"The doctor told me to rest at home since I tested positive for the flu."
In plain English: Flu is a serious viral infection that makes you feel very sick with fever, chills, and body aches.
"She stayed home from work because she had the flu."
Usage: Flu refers specifically to influenza, a contagious respiratory illness caused by viruses, rather than any generic fever or cold. Use it when describing symptoms like high fever, body aches, and sudden onset that distinguish it from the common cold.
The word flu is a shortened form of influenza that entered common usage as a convenient way to refer to the illness without saying its full name. It traveled into English directly from Latin through medical terminology before being clipped for everyday speech.