Where did that information come from Edgy Name? is it Official i.e taken from a Book or Fan Made?
From what i can see to answer the OP....
I will assume after the Juggernaught Crashed onto the Hillside that had Water Streaming down it, some of the Urns in the Cargo Hold that had not been Primed/Weaponized for Bombardment.. had leaked into/fell into the Water and infected the Water, or overtime some left over Urns in the Cargo Hold infected the Water... (most likely the last point)
Overtime the Stagnant Water would lead to Moss, and Mold and thus Spores that are then infected by the Black Goo to create those Spores we see in the movie.
The Spores Release a Hybrid Fungi/Mold Spores that are infected with the Black Goo DNA.... these Spores then bury into a Hosts Blood Stream and start to fuse together and take DNA from the Host that allows these Spores to then start to merge and divide and start to take on the Hosts DNA to create a Hybrid Creature that then grows and bursts out of the Host.
But i guess when the books of concept art comes out we may get a better idea.... as the other option is David created these and then they just spread out over the landscape.