Hello there and welcome to BYC!
Sounds like you are enjoying this adventure!
Chicks between the ages of 2 to 12 week are still developing their immune systems. As they age their bodies learn to become immune to things in their environment. These are critical weeks for babies, so it is important to keep their environment as clean as possible and offer up the best of food, water and things that help to develop a growing immune system.
Coccidiosis is a protozoa present in all birds intestinal tracts. As they grow, they learn to deal with this protozoa and once they develop immunity to it, they rarely have trouble with it unless they are moved to new grounds where they again have to learn to develop immunity to this new area. Cocci is spread through the pooping and because chicks are so crammed in together in brooders, they are highly susceptible to dying form this protozoa. Cleanliness is a huge must to keep the chicks from becoming overwhelmed with this stuff. I raise all babies on wire in the brooder. The poop falls through the wire, they come into contact with just enough of it to develop immunity and nobody dies. You might consider raising them on medicated feed which contains Amprolium. This is a Thiamine blocker only and will stop the Cocci from reproducing. Corid is a brand name for and Amprolium product you can put in the water to stop an outbreak of Cocci which you can get at the feed store. I also HIGHLY recommend using Probiotics in all chicks water and even adult chickens water. I use it several times a week to help keep the immune systems pumped up all throughout their lives.
And generally speaking, Coccidiosis IS the number one cause of chick deaths.
It is also important not to over crowd them to the point of overheating or being run off from food and water. This is a big one...not getting enough to eat or drink. So always provide a couple of areas to eat and drink. Never cover a brooder with anything but a screen or wire to allow for good oxygen and heat exchange and get the chicks out for field trips to the living room floor and to a fenced off area outside daily. They need outside exposure as well to develope immunity to new grounds early on so when you take them outside, they have already picked up a few "things" from the out of doors.
Definitely stop by our learning center as CTKen has suggested. All kinds of great articles on all the aspects of raising poultry.
So make yourself at home here and if you have any questions, feel free to as around the forums.
Welcome to our flock! :-)