Community-Acquired Pneumonia (CAP): Given the symptoms of dry cough, fever for 5 days, difficulty breathing, and generalized rhonchi, CAP is a highly probable diagnosis. The absence of a raised total white cell count does not rule out pneumonia, especially in elderly patients who may have a blunted immune response.

Acute Bronchitis: This could be a consideration given the cough and rhonchi, but the presence of fever and difficulty breathing leans more towards a lower respiratory tract infection like pneumonia.
Influenza: Although the total white cell count is not raised, influenza can present with similar symptoms, especially during flu season. The absence of a high white cell count does not rule out influenza.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: If the patient has undiagnosed COPD, an exacerbation could present with these symptoms. However, the question states no known medical illness, making this less likely.

Pulmonary Embolism (PE): Although less likely given the symptoms, PE can present with difficulty breathing and cough. It's crucial to consider PE due to its high mortality rate if missed.
Tuberculosis (TB): Especially in elderly patients, TB can present with non-specific symptoms like cough and fever. It's essential to consider TB, especially if the patient has risk factors or has been exposed.
Cardiac Causes (e.g., Heart Failure): Difficulty breathing could be a sign of heart failure, especially if the patient has underlying cardiac disease not mentioned in the history.

Pneumocystis jirovecii Pneumonia (PCP): Typically seen in immunocompromised patients, but could be considered if the patient has an undiagnosed immunodeficiency.
Interstitial Lung Disease: Certain types could present acutely, but this would be less common and might require further investigation to diagnose.
Lung Cancer: Although rare as an acute presentation, lung cancer could cause these symptoms, especially if there's an associated pneumonia or obstructive pneumonia.