What other pandemics has the world faced starting with the 20th century?

Since 1918, the world has experienced three additional pandemics, in 1957, 1968, and most recently in 2009. These subsequent pandemics were less severe and caused considerably lower mortality rates than the 1918 pandemic.2,3,4 The 1957 H2N2 pandemic and the 1968 H3N2 pandemic each resulted in an estimated 1 million global deaths, while the 2009 H1N1 pandemic resulted in fewer than 0.3 million deaths in its first year. This perhaps begs the question of whether a high severity pandemic on the scale of 1918 could occur in modern times.

Many experts think so. One virus in particular has garnered international attention and concern: the avian influenza A(H7N9) virus from China. The H7N9 virus has so far caused 1,568 human infections in China with a case-fatality proportion of about 39% since 2013. However, it has not gained the capability to spread quickly and efficiently between people. If it did, experts believe it could result in a pandemic with severity comparable to the 1918 pandemic. So far, it has shown only limited ability to spread between people. Most human infections with this virus have result from exposure to birds.

When considering the potential for a modern era high severity pandemic, it is important; however, to reflect on the considerable medical, scientific and societal advancements that have occurred since 1918, while recognizing that there are a number of ways that global preparations for the next pandemic still warrant improvement.

Source: https://www.cdc.gov/flu/pandemic-resources/reconstruction-1918-virus.html