Hello, Thanks for posting your query. Considering the details you have provided its most likely that you are suffering from a nosocomial (hospital acquired )infection with serrratia marcescens. The fact that damp moist conditions predispose to such infections and that you are a diver further increase your susceptibility .These bacteria are known to cause pneumonia which are resistant to many antibiotics and if your culture is sensitive to bactrim then your treatment started is appropriate . Though there is no consolidation seen I would consider it a lower respiratory tract infection with serrratia rather than an incidental finding . The infection can spread through many routes ( airborne , gastrointestinal etc ) however hand to hand transmission from healthcare workers is the major cause for its spread . Also spread is more likely before the symptoms have developed . As of now you can practice good hand hygiene and universal precautions to prevent the spread of the infection. To get it completely cured you might need additional parenteral antibiotic like amikacin if its susceptible . Your doctor might repeat your investigations to ensure eradication of the bacteria . It s advisable to avoid diving till you recover completely . Hope I have answered your query. I ll be available for any follow up queries. Regards