Influenza can cause swelling in the lung tissue. This is different from pneumonia, in that it is not fluid in the lung air spaces and different from bronchitis, where there is swelling and inflammation along the tissues lining the lungs.
In really severe influenzal lung swelling, the air spaces swell shut, the lungs are bleeding and bruised, and the person dies. Young healthy recruits in the 1917-1918 influenza would literally turn blue as they were no longer able to breathe and they would die.
If a person is still feeling exhausted after the initial week of influenza, they need testing to find out if they have lung swelling. This can be done at home or in your doctor's office.
To test at home, the patient should sit relaxed for 10-15 minutes. Take a one minute pulse count: normal is 60-100 beats in one minute. Then the patient should get up and walk until short of breath. Sit back down and repeat the pulse. If the pulse is jumping up 30 points or if it is over 100 after walking, there is still lung swelling. The treatment is rest.
To test in my office, I add a pulse oximeter. I get a resting oxygen and pulse level, walk the person and then watch the recovery. The oxygen level will often drop and then rise to the sitting baseline as the heart rate recovers. Most people do not need oxygen if they have a healthy heart and healthy lungs to start with.
You can see why influenza would be so dangerous to someone with an unhealthy heart or lungs, because the heart can’t make up the difference.
I had influenza in 2003 and had lung swelling to where I could not walk across the room without my heart rate going to 132. Sitting, my heart rate was 100. My normal heart rate is 65-75. It took two months for the swelling to subside and mostly I lay on the couch. Be reassured that if you rest when you need to, you will recover.