It does not need to be declared.
If you have a policy already in place for 5 years or more, there is no issue.
If you do not have a policy for 5 years but the condition arose (including signs & symptoms) after you took the policy, there is no issue
If you do not have a policy in place at all, then whether or not you declare it, it will not be covered. Any claim that would come into the health insurer will state the date when the signs & symptoms arose and the onset date of the condition & as they would indicate a date prior to the start date of the policy it will simply be deemed to be a Pre existing condition which holds no cover for 5 years.
If you need any clarity on this let me know!