The study aims to identify the general properties of emerging infectious agents that determine the success of two simple public health measures in controlling outbreaks: isolating symptomatic individuals and tracing and quarantining their contacts. The success of these measures is influenced by the proportion of transmission occurring before the onset of overt clinical symptoms (or via asymptomatic infection) and the inherent transmissibility of the agent (measured by the reproductive number \( R_0 \)). The authors estimate these quantities for four viruses: SARS-CoV, HIV, smallpox, and pandemic influenza. They find that SARS and smallpox are easier to control using these measures due to their low \( R_0 \) and \( \theta \) values. The study emphasizes the importance of direct estimation of the proportion of asymptomatic and presymptomatic infections through contact tracing during an outbreak of a novel infectious agent. The analysis also highlights the need for a more robust analytical framework to capture the impact of contact tracing and other reactive control policies on disease transmission dynamics.The study aims to identify the general properties of emerging infectious agents that determine the success of two simple public health measures in controlling outbreaks: isolating symptomatic individuals and tracing and quarantining their contacts. The success of these measures is influenced by the proportion of transmission occurring before the onset of overt clinical symptoms (or via asymptomatic infection) and the inherent transmissibility of the agent (measured by the reproductive number \( R_0 \)). The authors estimate these quantities for four viruses: SARS-CoV, HIV, smallpox, and pandemic influenza. They find that SARS and smallpox are easier to control using these measures due to their low \( R_0 \) and \( \theta \) values. The study emphasizes the importance of direct estimation of the proportion of asymptomatic and presymptomatic infections through contact tracing during an outbreak of a novel infectious agent. The analysis also highlights the need for a more robust analytical framework to capture the impact of contact tracing and other reactive control policies on disease transmission dynamics.