What virus causes fluid around lungs?

Pleural effusion. Lots of viruses (and also bacterial and fungi) can cause fluid around the lungs in the right situations. There are also non-infectious causes of fluid around the lungs. Finding the cause usually involves taking a sample of the fluid and sending it to the lab for analysis.
Many viruses can. Viruses such as adenovirus, coxsackieviruses, cytomegalovirus, epstein-barr virus, influenza, mumps, parainfluenza, respiratory syncytial virus, etc..., can cause inflammation of the lining of the lungs, and lead to fluid there. Of course, many people get infected with these viruses and just have milder illnesses... But the unlucky folks can become very ill.