The Spanish Flu, also known as the Influenza.
World War 2 diseases I am not familiar with, but the Spanish flu was an epidemic around the world at the end of World War 1.
(World War 2 1939-1945) In this time period, there was an epidemic of influenza from 1940-1941 in the United States. Also, there was an outbreak of polio immediately after the war, from about 1945-1949.
The terms used are epidemic or pandemic if the disease suddenly appears and spreads throughout the population (example plague or influenza). However some diseases (dental caries, arthritis, malaria, etc.) are wide spread and always present in many people. These are called endemic diseases.
There is a world-wide epidemic of chlamydia now.
The increasing population also increases the chances of a world epidemic. If there are more people, there are more chances that people could catch/spread a disease.
The flu epidemic.
LOTS of people got the flu. I mean LOTS.
Influenza. A worldwide flu epidemic broke out in 1918, which was also the last year of World War 1, but the epidemic killed more people than died during the entire war.
Epidemic. If it is world-wide, then it is called a pandemic.
Pneumonia
an escalating epidemic
Started the AIDs epidemic