Influenza: This is a common condition that matches all the symptoms provided, including fever, myalgia, cough, and can also cause gastrointestinal symptoms like abdominal pain and diarrhea, especially in children.

Gastroenteritis: Often caused by viral or bacterial infections, leading to symptoms like diarrhea, abdominal pain, fever, and sometimes myalgia. Cough might not be a primary symptom but can occur, especially if the infection affects the respiratory system as well.
Respiratory Syncytial Virus (RSV) Infection: Commonly affects children and can cause symptoms like cough, fever, and myalgia. Gastrointestinal symptoms can also occur, making it a plausible diagnosis.
Adenovirus Infection: Can cause a wide range of symptoms including fever, cough, myalgia, abdominal pain, and diarrhea, depending on the type of adenovirus.

Pneumonia: While cough and fever are more typical symptoms, some forms of pneumonia (like pneumococcal pneumonia) can present with abdominal pain and diarrhea, especially in children. Missing this diagnosis could be fatal.
Sepsis: Although less common, sepsis can present with any combination of these symptoms, especially fever, abdominal pain, and diarrhea. It's critical to consider sepsis due to its high mortality rate if not promptly treated.
Meningitis: Though typically presents with headache and stiff neck, in early stages or in young children, meningitis can manifest with nonspecific symptoms like fever, myalgia, and abdominal pain. It's a diagnosis that cannot be missed due to its severe consequences.

Leptospirosis: A bacterial infection that can cause a wide range of symptoms including fever, myalgia, cough, abdominal pain, and diarrhea. It's less common but should be considered in individuals with exposure to contaminated water.
Hantavirus: Can cause hantavirus pulmonary syndrome (HPS) or hemorrhagic fever with renal syndrome (HFRS), presenting with fever, myalgia, and gastrointestinal symptoms. It's rare but has a high mortality rate, making it important to consider in the right epidemiological context.
Lymphocytic Choriomeningitis (LCMV): A viral infection that can cause fever, myalgia, and abdominal pain, with less common symptoms including cough and diarrhea. It's rare and typically associated with exposure to rodents or their droppings.