Not necessarily. i don't have personal experience with coccidiosis, but lots with mites. i've had a pen where only one bird had a bad infestation. When i checked the others, i found none. So i think some birds are more susceptible to mites, just like some people get eaten alive by mosquitos and the person right next to them is fine. With most diseases, you will have some birds suffer or die and others pull through.
Although, if you find mites, lice, or have even one bird with cocci, you would want to treat them all.
Although, if you find mites, lice, or have even one bird with cocci, you would want to treat them all.