Fingernail question, help please!!!!?

What causes uneven nail surface (nails that look like ripples) ? And how can I prevent it? I usually have uneven nail surface on my thumb, index finger, and the middle finger. It's not very uneven, but i can see it when i paint my nails. I very healthy, i take vitamins, it can't be genetic, cause' none of my family members have that. And i have never had any injuries on my nails.