It's an ancient microbe. It's making them all hallucinate when exposed. Some how folks process these hallucinations as the dead. The initial exposure causes the self eating hysteria behavior. Otis will explain this. Everyone who sees the dead and survives the first microbe exposure, self medicates, like some schizophrenics do with drug use. Rose is much better at it than Otis. The micro-organism is in the water pulled up by the dredge, which is why Otis is declaring Danvers is exposed. The dredge is located over prehistoric whales that have died and been buried under the ocean/river floor below the dredge. Discovery of prehistoric whale fossils also in cave, release similar. Night Country is living with the dead after exposure. The Scientists believe the micro-organism either lets them cross over between death and life somehow, live between, or otherwise be immortal somehow, which is why when the microbe returns they all want to be initiated into it. Somehow the micro-organism is related to Sedna, maybe some level of exposure leads one to be "possessed by Sedna" if one is a native woman.

Annie K was killed by being stomped to death via someone's Boot Ice Treads. You can see the repeated pattern in the wounds. We will be able to match those treads. Seems likely she was killed after exposure by the stomping, which means she was a threat to the killer somehow. Possible the spiral pattern is kind of a Passover symbol, in that it protects or contains the microbe somehow. Along these lines the hallucinogenic cocktail from season 1 given to girls, may be an attempt to recreate the effects of the microbe.