Parental Fitness: What makes you a fit parent?
When the court has to determine custody, it looks at a number of factors. One of those factors is the physical and mental condition of the parties. Now, this doesn't mean that the court is going to perform a physical fitness test and expect you to do 20 pull ups and run a mile under eight minutes. Most of the time, this factor really isn't all that important, because the court isn't going to differentiate between two otherwise fit parents. This isn't a question of age or physical prowess.
What the court is really looking at is your ability to care for your children. Ask yourself, "Is ____ something that keeps me from being able to care adequately for my children?" If you suffer from a serious mental illness, this may affect your ability to effectively parent. By "serious disability," I don't mean depression. I mean a mental condition that seriously affects your ability to function in day-to-day life. A physical disability works basically the same way.
Before you get too upset, remember that, in most cases, custody isn't something that you either win or lose. In most cases where custody is contested, the worst case scenario is shared custody. Barring extenuating circumstances, the court won't keep your children away from you entirely. The question becomes not who will have primary physical custody of the child, but how you and your child's father will share custody.
The court will take your physical and mental condition into consideration, but the fact that you suffer from a mental illness or have some sort of physical disability doesn't mean that you'll lose custody and your child's father will win. All it means is that the court will take your condition (and his) into consideration when determining what an appropriate custody arrangement will be. The purpose of the court's involvement is to make a determination about what is in the best interests of the child.