Zhou Ruoran's illness wasn't very severe, but the real problem lay within the Gu Insect inside her body.
In the Cultivation World, there were also those who used Gu, but the tenacity of the Gu Insect inside Zhou Ruoran was something Ding Fan had never seen before.
It couldn't be killed by shock or driven out; it could only be raised within the body, allowing the Gu Insect to suck her blood. The blood in a human body is limited, and Zhou Ruoran would inevitably die from the Gu Insect draining all her blood.
Moreover, most fatally, the Gu Insect had several violent outbursts each day. Once the Gu Insect went berserk, the agony would make Zhou Ruoran wish for death. This was also why Zhou Ruoran had previously contemplated suicide.
Regarding the Gu Insect, Ding Fan didn't have much in the way of solutions; he could only use his True Qi to pacify it, trying to prevent it from going berserk.