Yu Xian's father isn't suffering from an illness but much worst than that. Her father is suffering from what we cultivators knew as 'Qi Deviation'.
There are different causes and effects of Qi Deviation but in her father's case, there is too much qi in his qi sea but instead of his qi sea being crippled, the qi in his qi sea became similar to poison in his body.
Yu Xian's father has a similar illness to food poisoning but in his case, it is qi poisoning caused by Qi Deviation.
Usually, a cultivator can cure that type of Qi Deviation by expelling the excess qi inside a cultivator's body or qi sea but Yu Xian's father's case is unusual.
He is suffering from not one but two types of Qi Deviation. The first is qi poisoning and the second is qi flow reversal.
Qi flow reversal is when the qi flowing through the meridians of a cultivator is in reverse which is fatal to a cultivator's cultivation base and health.