Coccidia oocysts occur naturally in the soil and are part of the normal flora of a chickens gut in very small numbers. They multiply rapidly in wet conditions where the chickens pick up more oocysts and when this happens the population in the chickens gut can explode and cause disease.
Can you post pictures of the affected birds?
If the pullets are not willingly eating or drinking, you need to drench each bird. Give each bird 1/2 tsp undiluted Corid. CAREFULLY drip it along the corner of the beak so it wicks in and the bird swallows on its own. I would drench once daily for 3-5 days.
The Corid water for the initial period of treatment is 2 tsp per gallon of water for 7 days with no other source of drinking water available. Once you mix up a gallon of water and fill the waterer, any extra solution can be kept in the refrigerator. Change the water the next day.
After the first week, drop the concentration to 1 tsp per gallon of water and continue for another week.