You may already have a travel insurance policy, for example a policy attached to your bank account. Make sure you read the terms and conditions carefully. The policy may not cover pre-existing medical conditions. You should let the company know that you have a pre-existing condition as this may affect your cover.
Even if the company is already aware of your medical history, you should update them if your condition changes. If your medical condition is excluded from their standard cover, the bank may allow you to pay a top-up fee so that you are fully covered. This may be cheaper than buying travel insurance separately.