My 39 year old brother in law has been in the hospital for 3 days now with what they are calling viral endocarditis.  He has fluid around his heart and is requiring oxygen to help him breath.  He is currently getting lasix every 8 hours.  He had several ECHO's which also showed  his heart was enlarged.  He is not on any antibiotics which I am assuming is because they suspect it is viral.  Is this a common occurance in such a young person?

Summarized question: What causes Viral Endocarditis?