Causes scour usually in calves 5 – 25 days old. It can be moderate to severs. Calves can become dehydrated very quickly.
Cryptosporidium parvum. This can be shed from some cows in faeces, also already present in faecal matter on the farm.
Send a faecal sample to laboratory / vet for diagnosis.
Oral electrolyte therapy for dehydration and your vet will also need to prescribe medication.