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Coccidiosis is a protozoan parasite that may produce clinical symptoms in calves from 1 month to 1 year of age. The major damage to calves is the result of the rapid multiplication of the parasite in the intestinal wall and the subsequent rupture of the cells of the intestinal lining. Several stages of multiplication occur before the final stage, the oocyst (egg), is passed in the feces. Oocysts are extremely resistant to environmental stress and are difficult to remove from the environment completely. Oocysts must undergo a final process called sporulation before they are infective again. Oocysts frequently contaminate feed and water. When the sporulated oocysts are ingested by calves, they start their life cycle over in the new host. In mild cases, calves only have watery diarrhea, but in most cases, blood is present in the feces. Straining, along with rapid dehydration, weight loss and anorexia (off feed), may be evident.

Scours can be classified into two types, nutritional and infectious. Nutritional scours is usually caused by stress to the calf due to a breakdown in management routines. Nutritional scours often progresses to an infectious scour, which is caused by a high population of pathogens. The initial digestion of milk occurs in the abomasum (or fourth stomach) and this progresses further in the intestines. Scours can usually be traced back to a failure of adequate milk digestion in the abomasum. Nutritional scours is simply the end-result of an oversupply of lactose in the intestines, caused by milk moving too rapidly out of the abomasum, so it cannot be broken down quickly enough. Pathogens use excess lactose as a nutrient source to increase in numbers. The rate of lactose digestion is then further reduced as a result of damage to the intestinal walls by these pathogens. This damage also causes body fluids to leak into the gut, thereby increasing the rate at which the calf dehydrates.

Clostridium perfringens are Gram-positive, spore-forming, anaerobic bacteria that are very commonly found in many environments, including soil, water, poorly preserved feeds, contaminated or improperly thawed colostrum or milk, calf-housing environments, and the normal bovine intestinal tract. C. perfringens proliferate the gut, the bacteria secrete toxins that have profound effects not only on the local intestinal environment (causing damage to the intestinal lining), but throughout the body as well. Death occurs when high levels of these bacterial toxins enter the bloodstream, leading to inflammation, shock, and cardiac arrest. C. perfringens are not spread from calf to calf, but it is not uncommon for several calves in a group to be affected at the same time, due to similar exposure and management practices.