English: "Yes, I feel great, thanks!" or "No, I've got a cold."
French: "Oui, merci, je me sens plein(e) d'énergie !" or "Pas vraiment, je suis malade."
In French, <em>être en forme</em> means to feel fit, dynamic, ready for action. When you ask someone <em>"Est-ce que tu es en forme ?</em>" you are basically asking them if they're fine physically.
Examples where this question could be appropriate is when someone is coming back from sick leave, or about to perform a show or a sports trial.
