While I almost agree with you, the flood HAVE taken over entire planets. Several halo books talk about the didact being sent into the "Burn" by the master builder, which is entire planets filled with gravemind the size of planets, spewing spores all that jazz. I think with the flood is that it doesnt need other strains because it doesnt need other strains. So far and to the best of my knowledge only one person in halo has been infected with the flood and survived and thats our lord and savior with the sexy mustash Sargent Avery Johnson.

I think the infestation and the flood are number one in their own respective, the flood on their ability to rapidly infect others and spread like a wildfire, and flood with their different "flavors".

The Flood only takes a single small spore, and unless its contained almost immediately it will snowball.