Cytokine storm

A cytokine storm is an overreaction of the body's immune system. It can be deadly. It consists of a positive feedback loop between cytokines and immune cells.
It is believed that cytokine storms were responsible for many of the deaths during the 1918 influenza pandemic, which killed a disproportionate number of young adults. In this case, a healthy immune system may have been a liability rather than an asset. Preliminary research results from Hong Kong also suggest this as the probable reason for many deaths during the SARS epidemic in 2003. Human deaths from the bird flu H5N1 usually involve cytokine storms as well.