There is no future for a Virus if it kills its host too quickly or at all , so over time the virus will tend to mutate becoming less deadly and allow more time for it to be spread rather than killing its host quickly which prevents onward transmission . As more and more people are exposed and recover the reservoir of potential hosts diminishes and the explosion of the virus slows
A lot of the damage to the body in healthy adults comes from the bodies immune response going into overdrive to the viral attack called a cytokine storm
Cytokine storms have the potential to do significant damage to body tissues and organs. If a cytokine storm occurs in the lungs, for example, fluids and immune cells such as macrophages may accumulate and eventually fill the lungs with fluid blocking off the airways, potentially resulting in death.. This goes a long way to explain why the 1918 flu epidemic killed so many young adults who had good immune systems . There were no antibiotics or steroids to control the immune response back in 1918