There are three known occurrences, one being in the Arklay Laboratory during the Mansion Incident in 1998. Another one happened at the Umbrella Caucasus facility where over 90% of the research staff were "eliminated" (either infected from virus or killed at some point). In the Primary Contamination Zone, the survival rate was under 2%. A "BL-4" contamination alarm went off in the Tricell facility underneath the ruins of the ancient Ndipaya Kingdom, BL-4 likely meant "Biohazard Level 4."