Say you have a chronic disease, not something that physically affects you but could be passed on to your kids, when do you tell the significant other?

For more in depth, its something that can be taken care of via medication, kinda like diabetes, but also has the potential, left untreated to kill you. This could get passes on to your kids and even grandkids. So when does this become relevant information for the significant other but also something you don't want to scare them off with?
Say you have a chronic disease, not something that physically affects you but could be passed on to your kids, when do you tell the significant other?
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