The 1918 flu caused an abnormally high number of deaths, possibly due to it provoking a cytokine storm in the body. (The current H5N1 bird flu, also an Influenza A virus, has a similar effect.) After the Spanish flu infected lung cells it frequently led to overstimulation of the immune system via release of cytokines (a protein that invokes the immune response) into the lung tissue. This leads to extensive leukocyte migration towards the lungs, resulting in the destruction of lung cells and secretion of blood and mucus into the alveoli and airways. This makes it difficult for the patient to breathe and can result in suffocation. 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.