Gastroenteritis: This is a common condition characterized by inflammation of the gastrointestinal tract, often caused by viral or bacterial infections. The symptoms of fever, chills, and diarrhea align closely with gastroenteritis, making it a plausible initial consideration.

Urinary Tract Infection (UTI): While UTIs are more common in women, older men can also be affected, especially if they have prostatic hypertrophy. Symptoms can include fever, chills, and sometimes diarrhea due to the proximity of the urinary and gastrointestinal systems.
Diverticulitis: This condition involves inflammation of the diverticula in the digestive tract, which can cause fever, chills, and changes in bowel habits, including diarrhea.
Food Poisoning: Consuming contaminated food can lead to symptoms of fever, chills, and diarrhea, and is a common issue across all age groups.

Clostridioides difficile (C. diff) Infection: This bacterial infection can cause severe diarrhea and is particularly dangerous in older adults, especially those who have recently used antibiotics. Missing this diagnosis could lead to severe complications.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease and ulcerative colitis can present with diarrhea, fever, and chills. Although less common, a flare of IBD could be life-threatening if not managed properly.
Sepsis: Any infection that leads to sepsis, a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs, must be identified and treated promptly.

Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to diarrhea, fever, and chills. It's less common but should be considered, especially in individuals who have recently traveled or have been exposed to contaminated water.
Ischemic Bowel Disease: Reduced blood flow to the intestines can cause a range of symptoms, including diarrhea, fever, and abdominal pain. This condition is more rare but can be life-threatening and requires immediate medical attention.
Neutropenic Colitis: Also known as typhlitis, this condition involves inflammation of the cecum and is often seen in individuals with neutropenia (low white blood cell count), which can be due to chemotherapy, bone marrow disorders, or other causes. It's rare but critical to diagnose due to its high mortality rate if untreated.