they didnt understand the importance of quarantine back then. Doctors didnt even think it was necessary to wash their hands between patients...even after performing autopsies they would enter the maternity ward and use their same blood stained hands to assist women giving birth. No wonder there was so many disease outbreaks!
Our health is very much dependent on cleanliness. Medical science did not always know that.
Actually the cleanliness guidelines (although not as strict as they are today) were not that bad during the 1918 outbreak. They did use quarantine measures and even disinfectant regularly to combat the disease. They also moved patients who were more susceptible to the disease (like pneumonia patients) to separate rooms. Hell health practitioners even wore masks in all their dealings.
It wasn't up to present day standards, of course. Due to lack of technology and some lingering outdated beliefs. But it was built upon the germ theory, which maintained that germs cause illnesses and can be spread. And they did seem to know something about air travel of diseases.(Even if they were slightly off.) Because Public Health Officials at the time wanted to educate people about coughing and sneezing into their hand and the proper way to dispose of nasal discharge. They also tried to limit gatherings of people, to avoid the uninfected sharing the same air as the infected. They even aimed to teach people to wash their hands before eating and just hygiene in general.
The type of hygiene you are referring to was that of the 17 - 1800s and was swiftly dying by the time the 1918 outbreak occurred because of updated measures to hygiene procedure and a better understanding of how infectious diseases work.
The 1918 Influenza Pandemic: Response
Of course, they had limited knowledge at the time compared to today, but the outbreak contributed highly to the understanding of illnesses in general. And led to better preventative measures as well as better medical science practices.
It was most likely allowed to spread so quickly due to well...... WWI.
Also, the Spanish Flu was such that even if you were healthy and clean as a whistle you would be ****ed over by it. It attacked healthy immune systems far better than it did even unhealthy ones. Not all viruses need us to be dirty or even unhealthy to be infected by them. Some of them just need to be foreign.