According to reports, the Clay Virus began to leak in an accident and seemed to initially be an airborne strain, but later became a transdermal strain. This was discovered from documentation indicating that hazmat suits were required for the Clay Virus ε-strain during the early days of the incident, that two organisms preyed on infected individuals and became infected during the middle part of the incident, and that S.T.A.R.S. personnel were not infected even when injured by Zombies and other infected individuals during the later part of the incident. The Clay Virus ε-strain virus presumably has a property to evolve with the progress of time and the external environment, and over time, gradually stabilizes from the airborne ε-strain virus, infecting and multiplying within host organisms and becoming a transdermal ε-strain variant. It can be said that the Clay Virus is a type that gradually grows less infectious over time.