Datatsushi's immature form from the 7th century (which the starved villagers ate) looked a bit like a macroscopic Ebola virus. With their bleeding eyes and mouths, what disease do the Shibito appear to be exhibiting symptoms of...?
Back at the start of the game, Tamon warns Yoriko to not eat or drink anything, referencing the various myths where eating the food of the afterlife prevents you from ever leaving. Harumi is never seen being exposed to the red water: she never bathes in it nor drink it, and she is never wounded by a Shibitonote Shiro's research explains that Shibito have red water instead of blood, which means that being wounded by a Shibito could possibly infect you. because, since she's a One-Hit-Point Wonder, the player must complete all her levels without letting her being caught or hit even once). That would explain why she's the only playable character who managed to escape the otherworld.
Additionally, the archive items reveal that she was born with minor psychic powers like the ability to have prophetic dreams, which explains why she can still sightjack despite not having been exposed to the red water.
