Jilleroo is right, a calf can't have coccidiosis before 3 weeks of age, as it takes that long for the bug to complete it's life cycle after ingested.
Scours doesn't necessarily mean infection. Osmotic diarrhea happens when being overfed, or poor milk replacer is used and the calf can't absorb it all. Undigested milk replacer goes to the large intestine and pulls fluid out of the body. The milk replacer can ferment down there and cause irritation of the lining, which can result in the blood spots you mentioned. The irritated areas are also more susceptible to infection by a number of bugs waiting for an advantage.
Poor quality milk replacers have ingredients which aren't digested as well, which can cause problems. They also don't form a curd in the stomach as well, allowing nutrients to pass too quickly down the intestines. A calf can't digest non-milk ingredients until 3 weeks of age, so milk replacers should contain ingredients from milk.
As others have said, ignore the scours if calf is perky. If anything, make sure you are mixing and feeding properly. You can do more harm than good with treatment. Antibiotic boluses can upset the normal bacteria in the gut making the calf more susceptible to worse bugs.
Unless you like playing vet and spending money on an assortment of drugs, at some point you have to focus on PREVENTION. If you do everything wrong, there are several diseases waiting to cause illness and death – which ones they are doesn't really matter.
Success starts with getting calves from a good source – ones that for sure got adequate colostrum. They should also not be shipped long distances while not eating or drinking. Without nutrition, calves can't have an immune system.
Calves should be kept separated without direct contact so they can't spread bugs to each other. Pens should be cleaned and sanitized between calves, and cleaned regularly to eliminate manure as a source of disease.
You also need to wash well your hands, any bottles, nipples, boots, etc, to keep from spreading bugs.
A few references.
http://books.google.com/books?id=Kqw2gC ... er&f=false
http://babcock.wisc.edu/sites/default/f ... _29.en.pdf