I have been working with my current employer for 7 months. I am an interim employee and currently on project that is ending in December. I have an 'indefinite' contract. (I'm in The Netherlands)
About a year or 2 ago I started feeling under the weather from time to time. I visited the doctor recently and after multiple appointments and tests we figured out I have a chronic illness. It's a disease that causes 'flares', which come and go. I could go months without an issue, I could potentially have issues for months. There's no way to know or predict this. So far it has been very doable with the right medication and in those 7 months I have only had to call in sick for 1 day.
Because I work on a project and doctors appointments are either made before work or on remote working days, my employer doesn't know about any of this. I will get an official statement from my doctor in 3 weeks, when my first medication plan is in.
Is it better to be open about this news? Do I tell him now, or in 3 weeks? Should I wait until it causes actual issues in my working days?