Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of runny nose and flu-like symptoms, often resulting from viruses such as rhinovirus, coronavirus, or influenza. The symptoms typically include sneezing, runny or stuffy nose, and sometimes fever, which aligns with the patient's presentation.

Allergic Rhinitis: This condition can cause similar symptoms, including a runny nose, sneezing, and nasal congestion, especially if the patient is exposed to allergens like pollen, dust mites, or pet dander.
Influenza: While considered under viral URTI, influenza (the flu) is worth highlighting separately due to its potential severity and the need for specific antiviral treatment in high-risk patients or during outbreaks.
Sinusitis: This can be acute or chronic and is often caused by bacterial infection following a viral URTI. Symptoms include facial pain, nasal congestion, and a runny nose, which may be purulent.

Bacterial Meningitis: Although rare, bacterial meningitis is a life-threatening condition that can initially present with flu-like symptoms, including a runny nose if there's associated sinusitis. Early diagnosis and treatment are crucial.
Pneumonia: Can present with flu-like symptoms and may progress to a severe respiratory illness. Certain types, like pneumococcal pneumonia, can be particularly dangerous in vulnerable populations.
COVID-19: Given its global impact and potential for severe disease, COVID-19 should be considered, especially if the patient has been exposed or is in a high-risk area.

Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can cause sinusitis, runny nose, and other systemic symptoms.
Sarcoidosis: Can cause sinusitis and other respiratory symptoms, though it's less common and typically presents with other systemic findings.
Leptospirosis: A zoonotic infection that can cause flu-like symptoms, including a runny nose, and is more common in individuals exposed to contaminated water or soil.