In French, I am sick is “Je suis malade.” A common expression is “malade comme un chien”, meaning “sick as a dog.” (Yes, it’s said in English sometimes as well, but everyone says it in French.)
Je suis malade depuis hier, je crois que c’est la grippe. – I’ve been sick since yesterday. I think it’s the flu.
Je ne peux pas aller au bureau, je suis malade comme un chien! – I can’t go to the office. I am as sick as a dog!
When I moved to the US, I was always surprised how frequently people would say they are “so sorry” when you’re sick. You don’t need to know how to say that because French people won’t be sorry! Instead, we would say something like “Soigne-toi bien,” which is literally “Treat yourself well” OR “Bon rétablissement,” which means “Good recovery.”
Tu viens ce soir? – Are you coming tonight?
Non, je suis malade. – No, I am sick.
Ok, soigne-toi bien! – Ok, treat yourself well!