I need to know why 'a' is omitted before headache in the below sentence.
Although he has undergone a very costly treatment, he has not been cured of a headache.
Correct - cured of headache.
I need to know why 'a' is omitted before headache in the below sentence.
Although he has undergone a very costly treatment, he has not been cured of a headache.
Correct - cured of headache.
I can't find a dictionary mentioning it but there is little doubt that "headache" is both countable and uncountable, which explains the use with no article in a non-plural context.
(Lexico) (not mentioned) You will find one instance of uncountable use.
(OALD) (not mentioned, which according to this dictionary is an indication that the word is only countable; it could be an omission.)