How did the flood in halo start?

The flood are like a mutant disease that infect living or dead individuals in the Halo series.

The Flood is a form of parasite, which can infect living people and aliens to turn them into zombie-like monsters. For more information, go to www.halopedian.com and search for flood in the search...

Halo started when The Covenant attacked UNSC battle ships. The UNSC had to flee because The covenant was to strong. The UNSC later discovered that The Covenant want to use an old weapon called The...

its basically a disease that mutates and takes over your body

You have got to tell me which flood you are talking about.