It is caused by a number of nematodal coccidia which all chickens are normally in contact with. However when conditions become damp and musty, excessive levels of these coccidia infect the caecal tubes if the chicks resulting in sudden death. An important symptom is blood around the vent or bloody diarrhea. Once noticed, swift action is needed to prevent high losses. Treatment is by administration of sulpha drugs.
Fast recovery occurs after treatment begins.
Obviously there are many more diseases which can infect chickens, but these are the more common ones the backyard chicken breeder will encounter. Unfortunately, many different disease of chickens show identical symptoms which makes accurate diagnosis very difficult. Sometimes, unavoidably, all birds in a flock may have to be destroyed and restocking not carried out for many months. In summary, remember that good husbandry is the best defence against disease in your flock but should an outbreak occur, swift and decisive intervention is required to minimise the ill effects. More detailed information can be found here .