When people get avian flu (or any influenza), the virus leaves the lungs and immune system weak and damaged, making them susceptible to secondary infection by other pathogens, especially if the person’s immune system was already weak before the influenza infection.
Bacterial pneumonia is an especially common secondary infection after influenza, because influenza weakens the lungs, which is where pneumonia attacks. In fact, when people die after getting influenza, they usually die from the secondary pneumonia infection, not the influenza itself.
When viruses infect the body, certain cells naturally produce chemicals called cytokines. Cytokines activate the immune system to respond to pathogens, but they also cause inflammation.
H5N1 avian flu viruses seem to cause the body to produce abnormal amounts of cytokines, more than ten times the amount usually produced in a regular influenza infection. This abnormal amount of cytokine is sometimes called a “cytokine storm”. Even though the large amount of cytokines activate the immune system to fight the viruses, the cytokines themselves harm the body, causing severe inflammation in the lungs. The severe inflammation can cause a person to be unable to breathe (acute respiratory distress syndrome), which often leads to death.
People with very healthy and strong immune systems are actually more susceptible to a cytokine storm, because they are able to produce more cytokines than people with weaker immune systems. Their strong immune system is like a double edged sword in avian flu infections - it is more able to fight off viruses, but it can also be more deadly when it causes a cytokine storm.