Sickness you have to act quickly to cure? (5,5)

'sickness you have' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'act quickly to cure?' is the wordplay.
I cannot really see how this works, but
'act' could be 'stage' (stage is a kind of act) and 'stage' is present in the answer.
The remaining letters 'fever' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'to' is the link.

Can you help me to learn more?