I know alot of you know this stuff already, but it is the most common killer of chicks up to 16 weeks, and can still affect some older chickens as well. There are 9 different species of cocci that are common to infect chickens. There are coccidia spores/protozoa everywhere. 4 are mostly deadly and pretty noticeable. The chick will look very sick; not eating, hunched up, MAY have blood in the stool, it's easy to see that they're sick. 5 are not as deadly, but harder to notice. They may not eat, or not eating enough. They tend not to be as active as the others. They may not come running for a treat like the others. OR not really be noticed at all. Some may recover on their own and just end up being behind the rest in growth. I just have one that for 2 weeks was less and less eager to come for the treat. She did sit more than the rest as well. From not eating enough, she became skinny. It "looked" like the others were beating her to the food, and she would stand away. The first symptom I noticed was that the others were growing bigger than her. Then I noticed that her crop was never full. I took her in and randomly put her on an anticoccidial (sulfadimethoxine). She spent the next day sleeping. The second day she stood all day and ate and drank a bit. Today she's eating and drinking, and getting active. Now I went outside and will treat the rest of them for coccidiosis. There is already another that just looks "off". Since coccidiosis is the most common ailment in young chickens, and some strains are pretty mild and not as noticeable, and may effect them over a longer period of time, and not have blood in the stool, a coccidiastat will be the first thing I use. I hope this is helpful .