BSL-4

A laboratory that contains the deadliest viruses known to mankind. These viruses are Ebola, Marburg, Lassa, Hanta, Smallpox, Crimean hemorrhagic fever, Argentinean and Bolivian hemorrhagic fevers. So any disease that makes you bleed to death, you should wear a positive pressure suit not to catch it.

Walking into a BSL-4 lab without a space suit is basically a suicidal act.