Wilson had information about the virus, which broke out in US military camps in Kansas at first. Wilson started sending the sick soldiers all over the place because he wanted us in WWI. He sent them all over the US and then all over the place in Europe including Spain (hence the name Spanish flu). He withheld info he had about the sickness to the people in the places he was sending them too, so they mixed even more. Like he sent a bunch of troops to Philadelphia and did not tell the mayor, and then the troops marched in a parade and then thousands more got sick.
The sickness was tricky because at first most people thought they just had the flu.
If Wilson had quarantined the sick soldiers, called back sick soldiers, and warned leaders in other places about it, the people working on vaccines would have gotten out in front of the outbreak. The virus would have started to mutate to a weaker form before it hit fresh populations.