I would rather have you ask such a question, (and it's a very sensible question) than not ask it! Many folks assume that putting their chicks on medicated feed, or keeping them on it long term will prevent coccidiosis. This is so not the case.
Here's an article that you might enjoy:
https://www.ars.usda.gov/northeast-...rasitic-diseases-laboratory/docs/coccidiosis/
The purpose of Amprolium is to block thiamine uptake in the protozoa. So, when the bird picks up cocci oocytes, they develop in the gut. Amprolium stops them from getting to the stage of multiplication, b/c they require thiamine to replicate. It's necessary for the bird to become exposed to the cocci so she can develop immunity.
This is why I give soil to my chicks within the first 2 weeks of hatch. A chick has her strongest immune system during those first 2 weeks. She gets antibodies passed from hen to egg. I continue giving them soil on a regular basis until they are freely moving from brooder to run at 3 - 5 weeks of age.
It's a rare event for broody hatched chicks to develop coccidiosis. Mama broody takes her chicks out into the yard shortly after hatch. She immediately has them scratching and digging in the soil. Some of their first snacks include feces laden soil, and they will even snack on Mama's poop! This is how their guts get popluated with a healthy mix of microbes. They even pick up a good load of cocci and other pathogens in this process. But they don't get sick b/c
the beneficial microbes in the soil outnumber the pathogens. They were created to have an immune system in place at hatch that will be strengthened by early exposure to the pathogens they will be encountering.
Now, I'm gonna climb on my soap box for just a moment longer:
The beneficial microbes in the soil outnumber the pathogens. IMO, one of the very best things a flock owner can do to improve the health of her flock is to ensure that the run is a healthy soil environment. Soil was not created to be bare. Healthy soil has a covering over it. It should either be covered with vegetation or mulch. A bare soil, fecal impacted chicken run is a run that is covered with unhealthy soil. The chemical balance of the soil becomes so overloaded with nutrients that plants CAN'T grow in such soil. I strongly advocate that a run that is not covered with vegetation should be covered with a deep composting litter. This protects the soil from becoming overloaded with N and P. The deep mulch provides a healthy environment for beneficial micro-organisms (bacteria and fungi). Those organisms break down the chicken feces, keep the nutrient levels in balance, and feed the beneficial insects and worms. The chickens benefit from having a ready source of beneficial bacteria and fungi to populate their guts, as well as lots of worms and insects. Their feed conversion rate is improved, the pathogens are kept in check, and they spend hours working through the litter, as they turn it into a nice fragrant compost.
Ok, I'll step back down off the soap box now.