The Spanish flu was the name given to a form of influenza (flu) caused by an H1N1 virus that started in some type of bird (avian origin).
The Spanish flu was a pandemic — a new influenza A virus that spread easily and infected people throughout the world.
Because the virus was new, very few people, if any, had some immunity to the disease.
Just like the flu we get today, the Spanish flu was particularly harmful to infants under age 5 and people over the age of 65. One thing that was different about the Spanish flu was that it also killed a large number of healthy adults, aged 20 to 40 years.