While keeping chickens isn't terribly difficult, sometimes they do have health problems or other issues that you can't figure out. Here are some common health problems you might see with laying hens or other poultry like turkeys, geese, and ducks.
Picking or Cannibalism
Sometimes chickens actually peck at each other's feathers and skin, a behavior called picking. If they draw blood, the problem can escalate, since chickens are attracted to the color red and to blood. Picking can even result in death. Some common causes of picking are overcrowding, a too-bright light left on too long, not enough food or water, or other stresses. So try to minimize stress, remove any injured or aggressive birds, and follow guidelines for lighting in your coop.
When eggs get broken in the nest box, hens may taste them. Once a hen tastes an egg, she will eat broken ones and even break eggs to eat them. So the best way to stop egg eating is to prevent it.
Roost mites are tiny bugs that drink your chickens' blood. If they grow unchecked, they can actually cause chicken illness and death. One way to know if you have mites is to inspect the eggs - tiny red spots that are about the size of a squished mite indicate their presence. You can also inspect the underside of the roosts for mites. Chickens will refuse to lay in nest boxes infested with chicken mites, so that can be another sign.
To treat a bad infestation of mites, you may need to use permethrin, a broad-spectrum insecticide. Keeping the chicken coop generally sanitary will help prevent mites. Diatomaceous earth can be used to combat mites as well. Wood ashes and non-toxic enzyme-based lice and mite sprays may also work (one such spray is Poultry Protector).
Molting isn't a chicken problem, but for a new chicken owner, it can be disconcerting! Your lovely hens will look ragged and bare. Molting happens once per year (although sometimes twice), typically in the fall. It's when chickens shed their feathers and grow new ones. During this time, they will not lay eggs. Molting typically lasts 3 months but in some cases, it can be longer. There is nothing to do about molting - just wait for it to finish.
Hens go broody when they decide to hatch a nest or clutch of eggs, sitting on them and providing body warmth. Even if the eggs are not fertilized, she will not leave them without putting up a fuss (often hissing and pecking at you if you try to lift her). Sometimes farmers want chickens to go broody - this way they will have baby chicks to replace aging hens or to grow the flock. But if you want to eat the eggs, you don't want a broody hen.
To "break up" a broody hen, remove her from the rest of the flock and keep her isolated with food and water and no access to nesting boxes. You can also try putting a clutch of ice cubes under her instead of the eggs.
There are many different diseases that can affect chickens and it can be hard to diagnose them. The basic idea is that once you have an ill bird, you should remove it from the flock and isolate it, in case the disease is contagious. It will also protect her from picking (see above) by the rest of the flock.