Who are the precursors in Halo? Are they basically flood?
The Precursors and the Flood are not one and the same. The Precursors were a race of super-intelligent beings that were so technologically advanced that they were almost godlike. They were not limited to one physical form and could visit other dimensions and alternate realities. They had powers that not even the Forerunners could replicate. They were also capable of changing their physical form if they wanted to. The Precursors were the ones who created our galaxy and seeded it with life. When the Forerunners rebelled against them for favoring humanity over them, their revolt was so swift and deadly that most of the Precursors were dead before they even knew what happened. It was at this point that they went into hiding. This was done in various methods, with one of them being the transformation of the Precursors into a dust form that would eventually transform back into their original forms. However, this did not go as intended, and their dust form became the first seeds of the Flood infestation. The Flood are completely different, as they lack the godliness and extreme intelligence that the Precursors possessed. They also do not have the same powers, or technological advancement, or especially anatomy.
What are some ways that the Flood could be reintroduced to Halo?
Halo Infinite.In the E3 2018 trailer for Halo: Infinite we come across a number of details which suggest a possible return of the Flood.The first is that morse code can be heard through a radio in the trailer. Translated, the code says “SOS ZETA HALO GRD” (GRD stands for General Radio Distress).Zeta Halo is Installation 07, the last of the 12 original Halo Rings that predate the Forerunner-Flood War.The most interesting thing about this ring is that is was where Mendicant Bias, one of the most powerful Forerunner Ancillas (AI) controlled this ring at one point. It also contained the Primordial, the last Precursor. IT was on Installation 07 that the Forerunners had experimented on human test subjects in an attempt to find a cure for the Flood.This is likely the Source of the cave writing in the E3 traielr which reads “Enemy within.”TL;DR, Halo Infinite is not only the next major installment in the Halo franchise, it also gives the perfect setting for the return of the Flood and what we know so far about the game gives us good reason to expect it is the case.
What if the Flood from Halo (in gravemind form) was in Avengers: Infinity War? What if Thanos was infected at the beginning, resulting in the Flood gaining knowledge of the Infinity Stones?
We are all screwed even without knowledge of the Infinity stones the Flood are a biological weapon in their base form which quickly develop into a Gravemind /Keymind aka the reincarnation of the Primordial . A Precursor . The Flood are Precursor so basically pissed off biological form of a god race gone a bit insane by anger .