Spanish flu
Main article: 1918 flu pandemic
The 1918 flu caused an unusual number of deaths, possibly due to it causing a cytokine storm in the body.[6][7] (The current H5N1 bird flu, also an Influenza A virus, has a similar effect.)[8] The Spanish flu virus infected lung cells, leading to overstimulation of the immune system via release of cytokines into the lung tissue. This leads to extensive leukocyte migration towards the lungs, causing destruction of lung tissue and secretion of liquid into the organ. This makes it difficult for the patient to breathe. In contrast to other pandemics, which mostly kill the old and the very young, the 1918 pandemic killed unusual numbers of young adults, which may have been due to their healthy immune systems mounting a too-strong and damaging response to the infection.[9]