Norovirus or other viral gastroenteritis: This is likely due to the patient's recent travel and exposure to someone with diarrhea. Norovirus is highly contagious and commonly causes outbreaks in closed environments like travel groups.

Bacterial gastroenteritis (e.g., Salmonella, Campylobacter, E. coli): These pathogens can cause prolonged gastroenteritis, especially in elderly individuals with potentially compromised immune systems. The recent travel history and exposure to someone with diarrhea increase the likelihood.
Food poisoning: Given the travel context, the consumption of contaminated food or water is a plausible cause of gastroenteritis.
Medication-induced gastroenteritis: Elderly patients often have multiple medications, some of which can cause gastrointestinal side effects, including gastroenteritis.

Clostridioides difficile (C. diff) infection: Although less likely, C. diff is a critical diagnosis to consider, especially in elderly patients who may have been exposed to antibiotics recently. It can cause severe, life-threatening diarrhea.
Inflammatory bowel disease (IBD) flare: While less common as a new diagnosis in the elderly, a flare of existing IBD (Crohn's disease or ulcerative colitis) could present with similar symptoms and is crucial not to miss due to its significant impact on quality of life and potential for complications.
Gastrointestinal obstruction or malignancy: These conditions can present with chronic gastrointestinal symptoms and are critical to diagnose early due to their potential for severe outcomes.

Parasitic infections (e.g., Giardiasis): Less common in developed countries but possible, especially with travel history to endemic areas.
Ischemic bowel disease: This condition can cause chronic or intermittent gastrointestinal symptoms and is more common in elderly patients with vascular disease.

The workup should include:

Stool tests: For bacterial, viral, and parasitic pathogens, including norovirus and C. diff toxin.
Complete Blood Count (CBC): To assess for signs of infection or inflammation.
Electrolyte panel: To evaluate for electrolyte imbalances due to chronic diarrhea.
Endoscopy: If symptoms persist or if there's a suspicion of IBD, obstruction, or malignancy.
Imaging studies (e.g., CT abdomen): If there's suspicion of obstruction, malignancy, or other structural issues.
Medication review: To assess for potential medication-induced causes.
Travel and exposure history: Detailed questioning about recent travel, food intake, and exposures to others with similar symptoms.