From Wikipidia’s article on the 1918 Flu Pandemic (Emphases mine):
The influenza strain was unusual in that this pandemic killed many young adults and otherwise healthy victims; typical influenzas kill mostly infants (aged 0–2 years), the elderly, and the immunocompromised. Another oddity was that this influenza outbreak was widespread in summer and fall (in the Northern Hemisphere). Typically, influenza is worse in the winter months.
People without symptoms could be stricken suddenly and within hours be too weak to walk; many died the next day. Symptoms included a blue tint to the face and coughing up blood caused by severe obstruction of the lungs. In some cases, the virus caused an uncontrollable hemorrhaging that filled the lungs, and patients drowned in their body fluids (pneumonia). In others, the flu caused frequent loss of bowel control and the victim would die from losing critical intestinal lining and blood loss.
In fast-progressing cases, mortality was primarily from pneumonia, by virus-induced consolidation. Slower-progressing cases featured secondary bacterial pneumonias, and there may have been neural involvement that led to mental disorders in a minority of cases. Some deaths resulted from malnourishment and even animal attacks in overwhelmed communities.
Scientists have used tissue samples from frozen victims to reproduce the virus for study. Given the strain’s extreme virulence there has been controversy regarding the wisdom of such research. Among the conclusions of this research is that the virus kills via a cytokine storm (overreaction of the body’s immune system) which explains its unusually severe nature and the concentrated age profile of its victims. The strong immune systems of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults caused fewer deaths.
In Japan, 257,363 deaths were attributed to influenza by July 1919, giving an estimated 0.425% mortality rate, much lower than nearly all other Asian countries for which data are available. The Japanese government severely restricted maritime travel to and from the home islands when the pandemic struck.
In the Pacific, American Samoa and the French colony of New Caledonia  also succeeded in preventing even a single death from influenza through effective quarantines. In Australia, nearly 12,000 perished.
Civil libertarians are invited to take a few months off from knee jerk reactionism. Otherwise relax. Stress weakens the immune system…or is that what we want? Sure wouldn’t mind a Surgeon General to give us a few hints.