This article compares bio-security practices on a farm and in a laboratory in Hong Kong, showing the connection between the concern for protection against life forms as a threat, and the care taken of life forms as an environment. It describes the articulation between daily practices aimed at maintaining a living environment, and the sanitary crises that have revealed new threats such as avian flu and SARS. The author highlights the role of sentinels in a conception of life forms as viral populations.