The OP updated the thread in
POST#18 and let us know the chicks are doing great!
@SunriseChickers Welcome To BYC!
Let's clarify a few misconceptions. There are several strains of Coccidia that affect chickens. Depending on what you read/research, there are 9 strains that have been found and documented.
Bloody feces is caused by the
strain of Coccidia not severity, one strain E. Tenella which affects the Ceca causes bloody stools. IMHO, if a chicken has a Coccidiosis overload, regardless of strain, it would be severe - all strains can be deadly and do a great deal of damage in a short period of time.
It would be better to treat the entire brooder if there is an overload of Coccidia. If birds are too weak to eat/drink and need specialized care or if they are being picked on, then separation may be needed. All the chicks in the brooder are exposed and ingesting the shed oocysts (found in poop, soil and feces contaminated food/water), so best to treat all.
Coccidia is a
protozoa (parasite), not a
virus.
The link you provided is very informative, after you read that one again, here is another one you may want to read as well:
https://www.merckvetmanual.com/poultry/coccidiosis/overview-of-coccidiosis-in-poultry