A ten day old calf cannot be sick from coccidiosis as the incubation period is 17 to 21 days. Meaning that it takes about 3 weeks to get sick after ingesting the bug because it takes that long to go through the stages of it's life cycle.
What exactly did the vet test? You may have older calves infected and shedding the bug in the manure if that's what you had tested.
How old are the calves, how are they raised, etc?
Generally speaking, once a calf shows signs of coccidiosis, it's too late to treat for that bug, as it's almost done with it's life cycle. Most will get better on their own and be immune. Preventive treatment is used to prevent the disease in other calves which were probably exposed the same as the sick one. But that only works early after exposure. Yes, you put Corid in the water to do that. Severe cases may need supportive treatment.
But again, that's assuming you have coccidiosis, at least in the 10 day old calf, it cannot be.
Read from the merck manual:
http://www.merckvetmanual.com/mvm/index ... /21202.htm