I have an aerobic septic system and found a small snake in my shower this morning. What a wake-up! I assume since the shower door was shut, that he came up through the drain. How did the snake get through the septic system? How can I keep them out??

I would tend to doubt it came through the entire septic system. Was it small enough to fit through the shower drain grate? Check your drain line for breaks and/or leaks.