The avian-borne flu that resulted in 50 million deaths worldwide, the Spanish flu is theorized to have originated in China and been spread by Chinese laborers being transported by rail across Canada on their way to Europe. In North America, the flu first appeared in Kansas in early 1918 and was visible in Europe by spring. Wire service reports of a flu outbreak in Madrid in the spring of 1918 led to the pandemic being called the “Spanish flu.” By October, hundreds of thousands of Americans died and body storage scarcity hit crisis level. But the flu threat disappeared in the summer of 1919 when most of the infected had either developed immunities or died.
In an age of advanced medical technology and innovation, we can hardly imagine a time where the flu could be deadly. There are definitely still epidemics in our time, like for example the recent Ebola outbreak. However, the epidemics in the past involved far greater numbers of people.
Typhus fever is spread by lice. The war brought on poor sanitation that probably led to a higher density of lice and made the transmission more prevalent.
Typhus fever caused three million deaths in Russia alone.
As humans have spread across the world, so have infectious diseases. Even in this modern era, outbreaks are nearly constant, though not every outbreak reaches pandemic level as the COVID-19 has.
The original use of the word Quarantine was the act of anchoring a ship arrived in Venice, Italy, for 40 days before landing.
Infectiousness of any disease is measured by the reproduction number (R0, or R naught). For example, Smallpox has an R0 of 6 whereas Measles has an R0 of 16.
Vaccination, if available, and herd immunity can curb the spread of disease.
Big cities, with exploding population and traffic, can lead to the rapid spread of any infectious disease.
Epidemic is a term that is often broadly used to describe any problem that has grown out of control. An epidemic is defined as "an outbreak of a disease that occurs over a wide geographic area and affects an exceptionally high proportion of the population." An epidemic is an event in which a disease is actively spreading.