coccidiosis is one cause of scours. The coccidia destroy the gut lining, that's what causes the bleeding and why the calf will pass blood and tissue. The calf strains and strains due to the irritation in his gut, and won't eat because it hurts. The cow's udder will be full. Antibiotics will treat secondary infection but don't work on the coccidia themselves. Treat the calf with fluids (water and electrolytes) by stomach tube, esophageal feeder, or oral syringe, every 6 hours or so. If he is to weak to stand you can try injecting sterile i.v. fluids subcutaneously, injecting 30 - 40 c.c per site until the entire litre is injected. Giving kaopectate or pepto bismal by stomach tube may slow scours and help keep him alive until his gut begins to heal. Treat your other cattle before symptoms appear.
Cryptosporidiosis is a similiar disease. Calves from 5-20 days are most susceptible, especially if they recieved inadequate colostrum. It's treatment is the same as for coccidiosis. Be aware cryptosporidiosis can affect people as well.