[Maximo]

Both gastroenteritis and HGE can be deadly, especially to a small dog, although HGE is usually the more serious.

Gastroenteritis is a medical term referring to inflammation of the gastrointestinal tract, usually the stomach and intestines. It can be caused by infection with bacteria, viruses, parasites, or reactions to medications or new foods. It often involves abdominal discomfort or pain, diarrhea and/or vomiting.

From PetMD:

We don’t know what causes HGE. Theories vary between a bacterial infection of the gastrointestinal tract (Clostridium perfringens has been blamed) to adverse reactions to food or parasites. For some reason, young, small breed dogs are at highest risk, as are individuals who can be described as high-strung or stressed.
We do know what happens during a case of HGE, however. Even though it is not inflamed, the lining of the intestinal tract becomes very leaky. Fluid, protein, and red blood cells seep out of vessels within the intestinal wall. The body responds with a process known as splenic contraction. The spleen serves as a reservoir for red blood cells. When the body senses that more may be quickly needed, the spleen releases its reserves into circulation.

Read more here: Hemorrhagic Gastroenteritis in Dogs - Bloody Diarrhea in Dogs | petMD