Acute Gastroenteritis due to Viral or Bacterial Infection: This is the most likely diagnosis given the symptoms of diarrhea, nausea, abdominal pain, and the history of drinking tap water in Mexico, which may be contaminated. The fact that 4 other people on the trip are also sick supports a contagious cause.

Giardiasis: Caused by the Giardia lamblia parasite, often found in contaminated water. Symptoms include diarrhea, abdominal cramps, and weight loss, which align with the patient's presentation.
Cryptosporidiosis: Another waterborne parasitic infection that can cause diarrhea, abdominal pain, and nausea, especially in individuals who have consumed contaminated water.
Food Poisoning: Given the travel history and consumption of local food and water, food poisoning from bacterial sources like Salmonella, E. coli, or Campylobacter is a possibility.

Cholera: Although less common, cholera is a critical diagnosis to consider due to its high mortality rate if not treated promptly. It is caused by Vibrio cholerae, often found in contaminated food or water.
Amphetamine or Toxin Exposure: The episodes of diffuse facial swelling could suggest an allergic reaction or exposure to certain toxins. While less likely, this could be life-threatening and requires immediate attention.
Allergic Reaction to Food or Insect Bites: The facial swelling could also indicate an allergic reaction, which needs to be addressed to prevent severe outcomes like anaphylaxis.

Cyclosporiasis: Caused by the Cyclospora cayetanensis parasite, this infection can lead to prolonged diarrhea and is associated with consuming contaminated food or water.
Intestinal Parasites (e.g., Hookworm, Roundworm): While less common in travelers, these parasites can cause gastrointestinal symptoms and should be considered if other diagnoses are ruled out.

Stool tests for bacterial, viral, and parasitic infections (including Giardia and Cryptosporidium).
Complete Blood Count (CBC) to assess for signs of infection or inflammation.
Electrolyte panel to evaluate for dehydration and electrolyte imbalances.
Allergy testing or specific IgE levels if an allergic reaction is suspected.

Hydration: Ensure the patient is well-hydrated to manage diarrhea and prevent dehydration.
Antibiotics or Antiparasitics: Depending on the cause, antibiotics or antiparasitic medications may be prescribed.
Symptomatic Relief: Medications for nausea, abdominal pain, and diarrhea as needed.
Allergy Treatment: If an allergic reaction is identified, treatment with antihistamines or corticosteroids may be necessary.
Monitoring: Close monitoring for signs of severe disease, such as dehydration, electrolyte imbalances, or worsening of symptoms.