I would still tend to think that the guy probably meant to say "are you insane?" rather than "are you sick?", even if the latter is technically the correct translation.
I've only now noticed this message.
Sorry for the late answer.
I'd accept the suggestion of KKMD.. if there is a need for the right meaning of the translated text.
"Malsana" is used when somebody is sick and usually one needs a doctor to "get health back"...
For this 'phrase situation' world "freneza"
/Ĉu vi estas freneza?/ would be better solution.
The right meaning of the word "freneza" is like "crazy" /Are you crazy?/, but same as in the English, also in Esperanto this phrase could be use with that 'other sense' meaning when one does not speak and mean about really crazy person, but the word is used only 'figuratively'.