COVID-19 Pneumonia: Given the presence of ground glass opacities in bilateral lower lungs, along with a mild pleural effusion and atelectasis, COVID-19 pneumonia is a highly plausible diagnosis, especially in the context of the current pandemic. The symptoms and radiographic findings are consistent with the viral pneumonia caused by SARS-CoV-2.

Community-Acquired Pneumonia (CAP): Bacterial or viral pneumonias can present with similar radiographic findings, including ground glass opacities and pleural effusions. The presence of atelectasis also supports this diagnosis, as it can occur due to inflammatory exudates or decreased lung volumes.
Influenza Pneumonia: Similar to COVID-19, influenza can cause viral pneumonia with ground glass opacities on imaging. The clinical context, including the time of year and exposure history, would help differentiate this from COVID-19.
Pulmonary Edema: While typically presenting with more diffuse and bilateral infiltrates, pulmonary edema can sometimes appear as ground glass opacities, especially if it's not severe. The presence of a pleural effusion could support this diagnosis, particularly if there's a history of heart failure or other causes of fluid overload.

Pulmonary Embolism (PE): Although less likely given the radiographic pattern, PE can sometimes present with ground glass opacities, especially if there's associated pulmonary infarction. The presence of a pleural effusion could also be seen in PE, making it a critical diagnosis not to miss due to its high mortality if untreated.
Tuberculosis (TB): TB can present with a wide range of radiographic findings, including ground glass opacities, especially in its early or atypical forms. Given its potential for severe consequences if not treated, and its variability in presentation, TB should be considered, particularly in endemic areas or in patients with risk factors.

Interstitial Lung Disease (ILD): Certain types of ILD, such as cryptogenic organizing pneumonia or nonspecific interstitial pneumonia, can present with ground glass opacities. However, these conditions are less likely given the acute presentation and the presence of a pleural effusion.
Lymphangitic Carcinomatosis: This rare condition, characterized by the spread of cancer through the lymphatic vessels of the lung, can present with ground glass opacities and pleural effusions. It's an important consideration in patients with a known history of cancer.
Eosinophilic Pneumonia: This condition can present with ground glass opacities and pleural effusions, particularly in the context of an eosinophilic reaction to drugs, parasites, or other allergens. It's less common but should be considered in the appropriate clinical context.