Gastroenteritis: This is likely due to the combination of diarrhea, fatigue, headache, chills, and night sweats, which are common symptoms of a gastrointestinal infection. The lack of sick contacts does not rule out gastroenteritis, as it can be caused by a variety of pathogens including viruses, bacteria, and parasites, some of which may be acquired through contaminated food or water.

Inflammatory Bowel Disease (IBD) Flare: Although less common in a new presentation at 67, IBD (including Crohn's disease and ulcerative colitis) can present at any age and could explain the gastrointestinal symptoms along with systemic symptoms like fatigue and night sweats.
Diverticulitis: This condition, which involves inflammation of the diverticula in the wall of the colon, can cause diarrhea, abdominal pain, fever, and chills. It's more common in older adults and could fit the presentation.
Urinary Tract Infection (UTI): While more common in women, UTIs can occur in men, especially at older ages, and can cause systemic symptoms like fever, chills, and fatigue, along with urinary symptoms that might not be immediately apparent.

Clostridioides difficile (C. diff) Infection: Although the patient reports no new medications, which is a common risk factor for C. diff, it's still a critical diagnosis to consider due to its potential severity and the fact that community-acquired cases can occur.
Typhoid Fever: Caused by Salmonella Typhi, this infection can present with fever, headache, and abdominal pain, and although less common in developed countries, it's a critical diagnosis to consider due to its severity and the need for specific antibiotic treatment.
Lymphoma: Certain types of lymphoma can cause systemic symptoms like fever, night sweats, and weight loss, and while less likely, it's a diagnosis that would have significant implications for treatment and prognosis.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, and arthralgias, among other symptoms. It's very uncommon but can be fatal if not treated.
Brucellosis: A zoonotic infection that can cause fever, night sweats, and fatigue, among other symptoms. It's rare in most areas but could be considered in individuals with exposure to animals or certain foods.
Tuberculosis (TB): While TB can cause a variety of symptoms including fever, night sweats, and weight loss, gastrointestinal TB is less common and might present with diarrhea and abdominal pain. It's a rare consideration in this context but important in certain populations or with specific risk factors.