A 67 yo M w/ a PMHx of IVDU presents to the ED w/ chest pain and fever of 101.5. His initial HR is 135. A CXR is obtained and he is started on broad spectrum antibiotics. An hour later, he begins complaining of worsening shortness of breath, and is found to be tachypneic and hypoxic, w/ a RR of 40 and SpO2 of 85 %. A repeat CXR is obtained (shown at R below, initial CXR on L).  What's the diagnosis? (scroll down for answer)