You could be a carrier for any number of genetic diseases and not ever know it, and if you just happen to have a child with another carrier of the same disease, there is a 25% chance they will have it. Many of these diseases can be treated very well if caught on a newborn screening, but the majority of states don't have many genetic diseases on their newborn screening panel, so you won't have any idea until your child shows symptoms, and in most cases, by then it is too late.
Source: Me
I am a carrier for a rare terminal genetic disease called Krabbe Disease. Had two perfectly healthy children, then when my third child was 20 months old, he lost all of his abilities to walk, crawl, and even sit up unassisted in a matter of weeks. He is the 25% chance we didn't even know existed since no one in either of our families ever had the disease. Now, he is fighting for survival through a stem cell transplant to prolong his life.
He has a page we use to spread awareness for anyone interested in seeing his journey. It's called Prayers for Arthur, hope for a cure.