A cytokine storm is an overproduction of immune cells and their activating compounds (cytokines), which, in a flu infection, is often associated with a surge of activated immune cells into the lungs. The resulting lung inflammation and fluid buildup can lead to respiratory distress and can be contaminated by a secondary bacterial pneumonia -- often enhancing the mortality in patients.
This little-understood phenomenon is thought to occur in at least several types of infections and autoimmune conditions, but it appears to be particularly relevant in outbreaks of new flu variants. Cytokine storm is now seen as a likely major cause of mortality in the 1918-20 "Spanish flu" -- which killed more than 50 million people worldwide -- and the H1N1 "swine flu" and H5N1 "bird flu" of recent years. In these epidemics, the patients most likely to die were relatively young adults with apparently strong immune reactions to the infection -- whereas ordinary seasonal flu epidemics disproportionately affect the very young and the elderly.
Deadly immune 'storm' caused by emergent flu infections
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