Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of yellow nasal discharge and congestion, often accompanied by other symptoms like cough and sore throat. The presence of diarrhea could be due to a viral gastroenteritis, which can occur concurrently with URTI, especially in children.

Bacterial Sinusitis: Can cause yellow or green nasal discharge due to the infection of the sinuses. It often follows a viral URTI and can be accompanied by facial pain and fever.
Allergic Rhinitis: Though typically associated with clear nasal discharge, in some cases, especially if there's a secondary infection, the discharge can become yellow. Allergic rhinitis can also lead to congestion and, less commonly, gastrointestinal symptoms like diarrhea due to increased histamine release.
Influenza: Especially during flu season, influenza can present with respiratory symptoms including congestion and yellow nasal discharge, along with systemic symptoms like fever and body aches. Gastrointestinal symptoms such as diarrhea can also occur.

Pneumonia: While primarily a lower respiratory tract infection, pneumonia can present with upper respiratory symptoms like congestion and yellow nasal discharge, especially in its early stages. The presence of diarrhea could indicate a viral cause or a complication.
Meningitis: Though less common, meningitis can present with non-specific symptoms including fever, headache, and sometimes gastrointestinal symptoms like diarrhea. Early recognition is crucial as meningitis can be life-threatening.
Septicemia: Bacteremia or septicemia can cause a wide range of symptoms including fever, chills, and in severe cases, diarrhea and respiratory symptoms. It's a life-threatening condition that requires immediate medical attention.

Cystic Fibrosis: A genetic disorder that can cause chronic respiratory and digestive problems, including yellow nasal discharge and diarrhea due to pancreatic insufficiency.
Immunodeficiency Disorders: Conditions like HIV/AIDS or primary immunodeficiencies can lead to recurrent or severe infections, including those causing yellow nasal discharge and diarrhea.
Gastroesophageal Reflux Disease (GERD): Though more commonly associated with heartburn, in some cases, especially in infants, GERD can lead to respiratory symptoms due to aspiration of stomach contents, potentially causing yellow nasal discharge. Diarrhea can also be a symptom in some cases.