Viral Gastroenteritis: This condition is highly contagious and commonly presents with symptoms of high fever, nausea, vomiting, abdominal pain, and headache, making it a likely diagnosis given the broad range of symptoms.

Food Poisoning: Similar to viral gastroenteritis, food poisoning can cause a wide range of symptoms including high fever, nausea, vomiting, abdominal pain, and sometimes headache, depending on the causative agent.
Appendicitis: Although it primarily presents with abdominal pain, appendicitis can also cause fever, nausea, vomiting, and occasionally headache, especially if the appendix has ruptured.
Influenza: Influenza can present with high fever, headache, and body aches, and sometimes gastrointestinal symptoms like nausea and vomiting, especially in children.

Meningitis: Although less common, meningitis is a life-threatening condition that can present with fever, headache, and sometimes nausea and vomiting. It's crucial to consider this diagnosis to avoid missing a potentially fatal condition.
Septic Abdomen: Conditions like a perforated viscus or peritonitis can cause severe abdominal pain, fever, nausea, and vomiting. Early recognition is critical for survival.
Encephalitis: This inflammation of the brain can cause fever, headache, and sometimes gastrointestinal symptoms. It requires prompt diagnosis and treatment to prevent serious complications.

Typhoid Fever: Caused by Salmonella Typhi, this condition can present with fever, headache, abdominal pain, and sometimes gastrointestinal symptoms. It's more common in travelers to endemic areas.
Brain Abscess: A rare but serious condition that can cause fever, headache, and sometimes nausea and vomiting, depending on the abscess's location and size.
Intestinal Obstruction: While primarily causing abdominal pain and vomiting, intestinal obstruction can also lead to fever and headache in some cases, especially if there's associated bowel ischemia or necrosis.