IEC (Working Holiday) visa holders are required by Immigration Canada to have health insurance for the entire time they are in Canada. The health insurance must cover:
medical care;
hospitalization; and
repatriation.
You may be refused entry if you don’t have insurance. If your insurance policy is valid for less time than your expected stay in Canada, you may be issued a work permit that expires at the same time as your insurance.
Pre-existing condition means any medical condition that exists before the effective date of your insurance policy. 
Important note: Please provide additional information regarding ANY pre-existing medical conditions in order to help us recommend a plan that will be suitable for you.