Retain the water and put 1/8-1/4 cup of clorox in bottom and then dump same amount of vinegar while shower is warming up, then get in. Afterward stay in bathroom a bit. The released chlorine binds to the humidity and makes a mild acid that can be deep breathed and start breaking it all up.
Can do the same using a humidifier in bedroom, put bowl nearby and put the clorox, followed by vinegar (DO NOT SNIFF AT IT) in the far corner of the room, have the humidifier near your headboard. If you wake up later with a headache, add fresh air.
Alternate is a warm pool and if it has a high area where the humidity, warmth, and chlorine is high, sit there an hour and it will clear up flu symptoms completely. They come back a few hours later, but using the bedroom trick helps me sleep fairly well during infection.