Speaking with a sick person we ask
¿Qué tal? – How are you?
¿Cómo te encuentras? – How do you feel?
In Spain it is normal to show optimism, so people usually diminish their pain:
No me encuentro bien – I am not Ok
Me siento un poco mal – I don´t feel well
and try to show that they are getting better:
Estoy (mucho) mejor, gracias – I am (much) better, thanks
If you want to ask somebody about her/his illness, you can use
¿Qué tienes? – What symptoms do you have? / What desease do you have?
¿Qué te pasa? – What happens to you?
Finally you should wish him/her:
¡Que te mejores! – Get well!
A ver si te mejoras – Get better soon!