When you have the flu virus your lungs get build up of mucus and other fluids which can sometimes make it hard for you to breathe.
Most times the fluid build up in the lungs due to the flu just makes it a bit difficult to breathe and is not serious but sometimes the flu virus and cause too much buildup in the lungs which can be deadly if not treated.
Some flu viruses can kill people if it gets too severe or you have other health issues as well but sometimes the flu is just annoying and needs a week or two to go away.
If you continue having difficulty breathing with the flu or you really have trouble breathing call 911 or go to the emergency room.
Or if the flu hasn't gone away within a couple weeks or so.
In severe cases the fluid buildup in the lungs can make it difficult to breathe preventing oxygen from reaching the blood stream which can cause death or brain damage.