The "Spanish Flu" was an H1N1 avian virus, according to current research. The severity is suspected to be the result of cytokine storms affecting the immune system.
See Medscape.com article on Influenza Transmission, from American Journal of Epidemiology.
Person-to-person transmission seems to be low compared to seasonal influenza transmission.

As noted in previous posts, it is spread by contact with bodily fluids rather than through the air like Spanish Flu.
However, aid workers need masks and protective clothing, because patients coughs spread fluids to those in contact.