What environmental factors contributed to the spanish flu and its spread?

I think this might help.

Avian influenza (bird flu) has the potential to cause a healthcare crisis of unprecedented global dimensions. Many predict a global pandemic far worse than the 1918 Spanish Flu, which killed 40 million to 50 million people. A larger, denser global population, coupled with modern transportation systems of both goods and people, could result in a pandemic killing far more people worldwide. The consequences of such a pandemic would stretch beyond just public health. International relations, commerce, politics, travel, medicine, and economic and social infrastructures would be affected due to widespread infection and worldwide mitigation efforts.