Resident Evil: Revelations with the team at Capcom. The lengthy interview covers a wide range of topics, from the inspiration for the 3DS iterations of Resident Evil to precisely what the essence of Resident Evil is.
When asked this very question, the team mentioned a word coined internally at Capcom, "bioreal." Essentially, anything that happens in Resident Evil title has to make sense within the universe Capcom has built. For example, a zombie might be reanimated by a virus but not, as Iwata pointed out, by magic. That said, the team admitted they don't concern themselves too much with "biorealism," and often include more fantastic events as development draws on.
One of the more candid moments in the interview came courtesy of assistant producer Tsukasa Takenaka. "Capcom is always like that," he said, "we make the main events interesting and then fill in the gaps with forced logic." Even Iwata was taken aback, saying, "Ohh... You just come right out and say it?" Check out the full Iwata Asks article for more on Resident Evil: Revelations and its focus on bringing horror back to the series.