Conversation so far:
When did Chrisye get sick?
July 2005.
What was his ailments?
complaining of breathing difficulties.
What else was wrong?
he was diagnosed with lung cancer.
Was it treatable?
he underwent the first of six rounds of chemotherapy on.
Where did he do that?
Mount Elizabeth Hospital in Singapore.
Did he stop after the first round?
he underwent the first of six rounds of chemotherapy.
What happened after the first round?
Chrisye's health improved.
Did he get sick again?
his health was again in decline.

Question: When did that start?
Self-contained rewrite: When did Chrisye's start to decline again after being diagnosed with lung cancer?