By the early 1900s there was a sharp decline in diphtheria and influenza. With use of vaccination, smallpox was much less a threat. Areas were being cleaned up. The medicine sciences were learning the causes and treatments for some diseases. Yet, there would still be some major pandemics. In Hawaii and then San Francisco the citizens suffered a major bubonic plague, known as ’black death’ in 1900. It reoccurred again after the San Francisco earthquake of April 1906; all due to the fleas on rats that carried the plague.
During World War I, the Spanish Flu ran its course from 1918 to 1919 with 50 million peopled dead in just 18 months. In the United States some 675,000 people and 200,000 in the United Kingdom died, most were young adults (ages 18 to 30) who seemed to be affected by this pandemic. So if you found an ancestor who died young during this period that may be answer. Because there were so many deaths, there was not even room in the local newspapers to print their obituaries.
The Asian Flu hit from 1957 to 1958 with 70,000 death in U.S. and 2 million around the world.
Tuberculosis was still a major dreaded disease during the 20th century with 100 people killed around the world.
Two other unusual types of death, especially with younger people that were not epidemics, which could have affected our ancestors involved child births and alcohol. In the 19th and early 20th century, women died of infection during childbirth many times from the doctors and mid-wives assisting in delivery. In the 1920s, in United States, the Volstead Act of 1919 became law, better known as Prohibition. People many times then used industrial ethyl alcohol to produce a drinkable alcohol. Only the federal government actually poisoned the industrial alcohol in hopes of stopping its manufacture. That method backfired after some 10,000 people in the United States died from drinking denatured alcohol.
So these are just some of the epidemics of an infectious diseases and unusual conditions which had a major affect on our ancestors, something that is important in family history research.
Here is an additional list of major disease outbreaks including where and when during the 19th century:
1803 New York had Yellow Fever 1820-3 Nationwide "Fever" - started Schuylkill River and spread 1831-2 Nationwide Asiatic Cholera: brought by English emigrants 1832 NY City and other major cities had Cholera 1833 Columbus, OH had Cholera 1834 New York City Cholera 1837 Philadelphia had Typhus 1841 Nationwide Yellow Fever: especially severe in the south 1847 New Orleans had Yellow Fever 1847-8 Worldwide Influenza 1848-9 North America had Cholera 1849 New York had Cholera 1850 Nationwide Yellow Fever 1850 Alabama, New York had Cholera 1850-1 North America Influenza 1851 Coles Co., IL, The Great Plains,and Missouri had Cholera 1852 Nationwide Yellow Fever: with some 8,000 who died in New Orleans 1855 Nationwide Yellow Fever 1857-9 Worldwide Influenza: one of the greatest epidemics 1860-1 Pennsylvania had Smallpox 1865-73 Philadelphia, NY, Boston, New Orleans, Baltimore, Memphis, Washington DC had Smallpox and Cholera along with a series of recurring epidemics of; Typhus, Typhoid, Scarlet Fever and Yellow Fever. 1873-5 North America and Europe Influenza 1878 New Orleans Yellow Fever: last great epidemic 1885 Plymouth, PA had Typhoid 1886 Jacksonville, FL had Yellow Fever