But it still sucks.
Often you won't have any warning- you'll just go to check on them and, even if you checked on them thirty minutes before, one will be dead. But sometimes you can tell when a chick is starting to go downhill, and you don't have to sit idly by and wait for the chick to die.
The first and most obvious thing to do (usually) is to isolate the chick. If you have a chick that seems to be ill it is prudent to isolate that chick so he/she doesn't spread illness to the other chicks. HOWEVER, I don't always do this. I usually figure that, by the time symptoms are present, if it's contagious, the damage is done. And if the chick is going to die I would rather it do so surrounded by it's little brothers and sisters- I know that makes me sound like a sap, but I always think of sick and premature babies, and how they heal and thrive much more quickly if they have physical contact. Why should chicks be any different? However, besides containing the illness, there are good reasons to isolate. It allows you to monitor the chicks droppings, and you may be able to keep the chick closer to monitor symptoms and act more quickly.
The next most important things to do is get the chick to eat and drink (especially drink). You may have to dip the chicks beak in the water or use a dropper- do what you can. Offering nutrient dense soft foods may help- cooked (with no oil or salt) and mashed egg yolk is often recommended.
It's also recommended widely to add vitamins to the water or hand administer them to the sick chick. Often illness in chicks can be attributed to a vitamin deficiency. Many use Poly-vi-sol infant vitamins (the no iron formula), which are available at any pharmacy. I haven't tried them- when I have a chick turn up sick I add 1 tsp. of raw apple cider vinegar and a few drops of molasses per quart of water. Go super easy on the molasses- too much sugar can make the situation worse. The combination of the ACV and molasses provides much of the vitamins and trace minerals that you would get in poly-vi-sol or commercially prepared electrolyte solutions.
In fact, I have a chick on this treatment right now. As I mentioned before I have had higher mortality with this batch of chicks than I have in the past. Yesterday (or the day before? now I don't remember...) one of the chicks started having leg problems- he can't lift himself up onto his legs and his toes were curled under. He could get around by pushing with his legs and flapping his little wings, but it was a lot of work. I started putting molasses and ACV in the water and making sure he drank- he seems to be improving. He is eating and drinking, and his toes have un-curled. He still won't stand, though, so we shall see.
Coming Soon: I hope to add some more tips from other chicken owners to this post- I have a call out to other BYC members. If you are a chicken owner and would like to add to this, please post below!