COVID-19: Given the symptoms of fever, cough, breathlessness, and the HRCT findings of ground-glass opacities (GGO) and reticular shadows along with mediastinal lymphadenopathy, COVID-19 is a highly plausible diagnosis, especially considering the pandemic context and the age of the patient, which puts them in a higher risk category.

Community-Acquired Pneumonia (CAP): The presentation of fever, cough, and breathlessness could also suggest CAP, especially in an elderly individual. The HRCT findings could be consistent with bacterial or atypical pneumonia.
Influenza: Similar to COVID-19, influenza can present with fever, cough, and respiratory distress, and can lead to similar radiographic findings, especially in older adults or those with underlying health conditions.
Tuberculosis (TB): Although less common in some regions, TB can present with chronic cough, fever, and breathlessness, and can cause a variety of radiographic patterns including GGO and lymphadenopathy, particularly in immunocompromised or elderly patients.

Sarcoidosis: This condition can cause mediastinal lymphadenopathy and pulmonary infiltrates, including GGO and reticular patterns. It's crucial not to miss sarcoidosis due to its potential for significant morbidity if left untreated.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with mediastinal lymphadenopathy and pulmonary involvement, which might appear as GGO or reticular shadows on HRCT. Given the potential for aggressive disease, missing this diagnosis could have severe consequences.
Pulmonary Embolism: Although the HRCT findings described are not typical for pulmonary embolism, it's a condition that can present with acute onset of breathlessness and can be associated with fever if there's associated pulmonary infarction. It's critical to consider due to its high mortality rate if untreated.

Idiopathic Pulmonary Fibrosis (IPF): While IPF typically presents with a more chronic course and characteristic radiographic findings of usual interstitial pneumonia (UIP), it's conceivable that an acute exacerbation could present with GGO and rapid deterioration.
Eosinophilic Pneumonia: This rare condition can cause acute respiratory failure and has a variety of radiographic patterns, including GGO. It's often associated with eosinophilia and can be triggered by various factors, including drugs and parasites.
Cryptogenic Organizing Pneumonia (COP): COP can present with flu-like symptoms and radiographic findings that include GGO and consolidation. It's a diagnosis of exclusion but is important to consider due to its responsiveness to corticosteroid treatment.