Viral Gastroenteritis: This is a common condition that can be contracted while traveling, especially when consuming contaminated food or water. The symptoms of fever, chills, nausea, and vomiting are consistent with a viral gastrointestinal infection.

Food Poisoning: Given the recent travel history, food poisoning from consuming contaminated or undercooked food is a plausible diagnosis. Bacteria like Salmonella, E. coli, or Campylobacter could be responsible.
Traveler's Diarrhea: Although the primary symptom mentioned is vomiting, traveler's diarrhea often presents with a combination of diarrhea and vomiting, and is commonly acquired while traveling abroad due to bacterial, viral, or parasitic infections.
Influenza: With the symptoms of fever, chills, and nausea, influenza should be considered, especially if the travel occurred during flu season or if there was exposure to someone with the flu.

Malaria: Although less common in Spain and Portugal, if the traveler visited areas with known malaria transmission or had other exposures (e.g., through blood transfusion or sharing needles), malaria must be considered due to its severe consequences if left untreated.
Typhoid Fever: Caused by Salmonella Typhi, this infection can present with fever, headache, nausea, and vomiting. It's more common in travelers to certain areas, but given the symptoms, it should not be missed.
Viral Hemorrhagic Fever (VHF): While extremely rare, VHFs like Ebola or Lassa fever can present with similar symptoms. Recent travel history and potential exposures would guide the consideration of these diagnoses.

Leptospirosis: A bacterial infection that can be contracted through water or soil contaminated with the urine of infected animals. It's rare but can cause severe illness.
Brucellosis: Typically associated with exposure to infected animals or their products, brucellosis can cause fever, nausea, and vomiting. It's uncommon but should be considered with the right exposure history.
Q Fever: Caused by Coxiella burnetii, this bacterial infection can be acquired from infected animals or contaminated products. It's rare but can present with non-specific symptoms like fever and nausea.