The 1918 Influenza Pandemic spread to nearly every part of the world. Most of the victims were healthy young adults (unlike most Influenza outbreaks). This was due to the way the Influenza attacked the body. The strain of Influenza was declared a Category 5 pandemic (see below).
Most Influenza outbreaks kill infants (0-2) and the elderly. The virus kills via a cytokine storm, which explains its unusually severe nature and the unusual age profile of its victims (the virus caused an overreaction of the body's immune system—the strong immune systems of young adults ravaged the body, while the weaker immune systems of children and middle-aged adults caused fewer deaths). Once infected, many would lose the strength to walk within hours and would die the next day.
Known as the “Spanish Flu” (due to the severity of the flu in Spain, as well as the lack of press censorship due to the fact that Spain was not involved in WWI), the Influenza outbreak of 1918 has been compared to the Black Death. Modern estimates place the number of deaths worldwide at anywhere from 50-100 million (more than double the death toll of WWI).
The cause of the flu itself remains unknown today. Modern researchers have studied frozen samples at the Center for Disease Control (CDC) in Atlanta, Georgia. Through genetic sequencing, it has been determined that the flu was an avian flu that jumped directly from birds to humans. However, they cannot link it back to any known species of bird, leaving the cause of the flu a mystery.