Severe acute respiratory syndrome
The 1918 Spanish flu was the first of three flu pandemics caused by H1N1 influenza A virus; the most recent one was the 2009 swine flu pandemic. The 1977 Russian flu was also caused by H1N1 virus, but it mostly affected younger populations. The ongoing pandemic of COVID-19, which began in 2019 and is caused by Severe acute respiratory syndrome coronavirus 2, is the deadliest pandemic since Spanish flu due to its spread worldwide and high death toll.