Halo and Resident Evil are the same universe - T Virus is modified flood
I was reading the resident evil wiki on the T virus, as one does, and I see that the T Virus is modified from the Progenitor virus that was discovered in the mountains. It's speculated in universe that the progenitor virus is millions of yesrs old, possibly older.
What else is ancient and has zombie-like properties? Flood powder.
For those that don't know, the origin of the flood is Precursors, a race of god-like beings who fought the forerunner. As a result of the war, the precursors turned themselves into this powder and spread it through the galaxy. Forerunner put this in pet food, the pets turned into zombies, and thats how we got the firdt flood.
Sounds similar to the T Virus. Scientists discovered this ancient substance and modified it into the T Virus. Under my theory, this ancient substance, the Progenitor Virus, was Precursor powder. Except instead of putting it in pet food, they tinkered with it to create bioweapons.
Any differences can be chalked up to modifications done to turn progenitor into T virus, as well as these being very small scale infections. We never see infection forms in Resident Evil because the flood never advances to that state. Every outbreak in Resident Evil never got bigger than a city. In Halo, we were dealing with planet wide outbreaks (or leftovers from galaxy wide outbreaks). Resident Evil outbreaks are extremely tiny compared to Halo's. Halo CE, they took over the whole ring and only just started on a protogravemind. Afaik, no Resident Evil even comes close to thebone we see in CE.
Similarities, well it explains the classic zombie trope of why zombies don't attack each other despite being feral beasts. The flood are a hive mind. Further similarities are the zombies mutating into stronger versions. Attack and fail to kill a zombie and it becomes a licker. Combat forms slowly evolving. On that note, the Nemesis Project - they created a fully mutated combat form.
There's still major differences. Like I said, they could be the tinkering to turn powder into t virus. It still wouldn't be the true flood. And also, the 'standard combat form' is likely a game thing. I doubt they all mutate the exact same way. And lastly, Resident Evil "flood" are spread by body fluid and bites, not by infection forms. From the comics, we know Halo flood can spread by body fluid as well, it's just not as common as the infection forms and seems a bit weaker (the only people we see turn by spores are already compromised - suggesting the body can fight it off to a degree, but remember, the T Virus is modified).
In the hundreds of years between Resident Evil and Halo, the T Virus is covered up and/or destroyed with no record. Umbrella Corp has long since filed for bankruptcy and/or is absorbed by Weyland Yutani (I like the idea that Aliens is in this verse as well, but thats another story)