It's tricky, I had one that was sick and immediately started treatment on the group. Most in the brooder are rooster chicks so I removed the 2 pullets that were bright eyed and running around with the others as a precaution since I wanted the pullets to stay healthy.
Lost one roo chick in the brooder by the next day. Kept that group on treated food and water and during that week one of the pullets that was moved to the garage just started to seem not well. Would not open her eyes, acted like she just felt yucky and wanted to be alone in the corner. Her stool never looked bad or runny. She just looked like she felt crummy. Kept those two on treated food and water for a week and one morning checked on her and she was looking up and me and acting like a whole new girl. I kept her in that brooder until yesterday and moved her back to the main group. So, they don't always look sick at the same time and can spread it out over a longer period of time. You want to nip this in the bud and stop it, not let it linger.