久病成良醫

long illness makes the patient into a good doctor (idiom)

(long) time / (long) duration of time

my old illness / chronic condition

illness / CL:場|场[chang2] / disease / to fall ill / defect

surname Cheng

to succeed / to finish / to complete / to accomplish / to become / to turn into / to be all right / OK! / one tenth

good / very / very much

good doctor / skilled doctor

medical / medicine / doctor / to cure / to treat