Traveler's Diarrhea: This is the most likely diagnosis given the recent travel history to Spain and Portugal, and the symptoms of fever, chills, nausea, vomiting, diarrhea, and dark-colored stool. Traveler's diarrhea is often caused by bacterial pathogens such as Escherichia coli (E. coli), and the symptoms presented are consistent with this condition.

Food Poisoning: Given the symptoms of nausea, vomiting, diarrhea, and dark-colored stool, food poisoning is a plausible diagnosis. This could be due to the consumption of contaminated food or water during travel.
Viral Gastroenteritis: Viral infections such as norovirus or rotavirus could also cause the symptoms described, especially in a traveler who may have been exposed to different pathogens.
Giardiasis: This is an intestinal infection caused by the parasite Giardia lamblia, which can be contracted through contaminated water or food. It is a common cause of traveler's diarrhea and could explain the symptoms.

Typhoid Fever: Although less common in travelers to Spain and Portugal, typhoid fever is a serious condition that can present with fever, chills, nausea, vomiting, and diarrhea. It is caused by Salmonella Typhi and is typically contracted through contaminated food or water.
Cholera: While rare in travelers to these regions, cholera is a life-threatening condition that causes severe diarrhea and dehydration. It is essential to consider this diagnosis, especially if the patient has been exposed to contaminated food or water.
Inflammatory Bowel Disease (IBD) Exacerbation: If the patient has a pre-existing condition like Crohn's disease or ulcerative colitis, travel and changes in diet could exacerbate symptoms, mimicking an acute infection.

Cyclosporiasis: This is a parasitic infection caused by Cyclospora cayetanensis, which can cause prolonged diarrhea in travelers. It is less common but should be considered if other diagnoses are ruled out.
Brucellosis: Although rare, brucellosis is a zoonotic infection that can be contracted through contact with infected animals or their products (like unpasteurized dairy). It can cause fever, chills, and gastrointestinal symptoms.
Leptospirosis: This bacterial infection is contracted through contact with water contaminated with the urine of infected animals. It can cause a wide range of symptoms, including fever, chills, nausea, vomiting, and diarrhea, especially in individuals who have been swimming or wading in contaminated water during their travels.