Astigmatism isn't a disease. It's a description of (basically) the shape of the cornea, the clear part behind which you can see the blue/grey/brown...part.
If light rays traveled left to right, from the left side of the page to the right side of the page...and you drew a line down the middle of the page, about an inch long, or so....and the light rays passed through the line, the line being a regular house window glass thing, they wouldn't deviate.
If the vertical line was a "spherical" lens, then all the lines that hit it from the left would converge to a single point over on the right side. The distance from the lens to the point where they all meet at the same time is called the focal distance. The point is called the focal point.
Magnifying lenses bend light this way. So you can take a magnifying lens, go outside and focus the sun onto a piece of paper or a piece of wood or leaf or ?, and all the light rays from the sun will converge and meet at that point. There's enough energy there to cause a fire.
If you were to look at that curved surface, and had lines running through the center, sort of like a clock face, all the lines would have the same curve. The line from 12 to 6 would have the same curve as the ones from 8 to 2 or 10 to 4.
That's a spherical surface.
If you had a really flat magnifying glass, you'd hold the lens further away from the paper for it to focus the sun. That larger focal distance indicates that that lens is "weaker", as it doesn't bend the light rays as 'fast' or as sharply.
If you took a basketball, made of glass or a spherical glass ball of some sort, and chipped a piece of glass off of it, all the curves would be the same.
If you used a doughnut, made of glass...or a football made of glass, and chipped off a piece of it, there'd be two curves. The sharpe curve or really round curve or really small radius curve would bend light faster or closer to the lens. The flat part would bend light too, but it's focal distance would be further away. So if you tried to focus the sun with this lens with a really curved surface 90 degrees away from a flatter curved surface, you'd get two focal points, two focal distances.
So the image of the sun wouldn't be a sharp little dot. It'd be a long dot which would slowly move 90 degrees to another long dot. And in between those two focal points would be a little circle where the least amount of dispersion of the light rays occurs. This is called the circle of least confusion.
So because you have a little astigmatism, means you have a little astigmatism. It's a physical observation. You are 8 feet tall with green hair, and purple skin from the planet Zod. Well, everyone from Zod is like that. Some are taller, some shorter, some less green, some more green (hair thing) and some with purple skin and some with pink and some with really red skin. Astigmatism is just the shape. Not everyone has spherical eyes.
There are people with astigmatic corneas that when they go get their Rx for glasses, have NO astigmatism. The shape of the cornea measured on a keratometer indicates that there are two curves 90 degrees apart where one is fairly flat, and one more curved. They 'should' have an astigmatism as measured by their refraction. But these people don't. This is because the lens inside the eye has adjusted itself to counteract that corneal irregularity or corneal nonspherical form. (I find that just amazing).
Most of us still have SOME astigmatism. If you get it corrected in your glasses you get to see better.
If you don't, it won't worsen or betterin itself, it just is and will be...And it may get a little better or a little worse, but it won't change much, it just doesn't. Afterall, we're zods.