Pneumonia: Given the patient's presentation with generalized weakness, recent diagnosis of possible pneumonia, and current chest X-ray showing basilar opacities, pneumonia is a highly likely diagnosis. The elevated WBC count (74) also supports an infectious process.

Atelectasis: The chest X-ray findings of left greater than right basilar opacities could be due to atelectasis, especially in the context of generalized weakness which might lead to decreased mobility and poor respiratory effort.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Although not directly mentioned, the patient's age and presentation could be consistent with a COPD exacerbation, especially if the patient has a history of smoking or COPD.
Heart Failure: The basilar opacities on the chest X-ray could also represent pulmonary edema, suggesting heart failure as a possible diagnosis, particularly in an elderly patient with generalized weakness.

Sepsis: Given the high WBC count and the patient's age, sepsis is a critical diagnosis not to miss. Sepsis can present with non-specific symptoms such as generalized weakness and can rapidly progress to a life-threatening condition.
Pulmonary Embolism (PE): Although the chest X-ray does not specifically suggest PE, it is a diagnosis that can be easily missed and is potentially fatal. The patient's recent history of multiple ER visits and current presentation could mask a PE.
Malignancy: In an elderly patient with generalized weakness and abnormal chest X-ray findings, malignancy (either primary lung cancer or metastatic disease) should be considered, as it can present with non-specific symptoms and has a significant impact on treatment and prognosis.

Interstitial Lung Disease: This could be considered if the patient's symptoms and radiographic findings do not respond to typical treatments for pneumonia or other common conditions.
Lymphoma: Although less likely, lymphoma could present with systemic symptoms such as generalized weakness and abnormal chest X-ray findings, making it a rare but possible diagnosis to consider.
Tuberculosis (TB): Given the patient's age and the presence of basilar opacities, TB could be a rare consideration, especially if the patient has risk factors for TB exposure or a history suggestive of TB.