This study identifies factors that determine the effectiveness of public health measures in controlling infectious disease outbreaks. The two main measures are isolating symptomatic individuals and tracing and quarantining their contacts. The success of these measures depends on the proportion of transmission occurring before symptoms develop or via asymptomatic infection (denoted as θ), rather than the inherent transmissibility of the pathogen (measured by the basic reproduction number R₀). The study estimates θ and R₀ for four viruses: severe acute respiratory syndrome (SARS), HIV, smallpox, and pandemic influenza. It concludes that SARS and smallpox are easier to control using these measures due to their lower R₀ and θ values. The study emphasizes the importance of estimating θ through contact tracing during an outbreak. It also highlights the impact of delays in isolation and contact tracing on the effectiveness of these measures. The analysis shows that for diseases with high θ values, contact tracing is crucial to counteract delays in isolation. The study provides a framework for predicting the success of public health interventions based on R₀ and θ, and underscores the need for further research into the impact of contact tracing and other targeted control measures on disease transmission dynamics.This study identifies factors that determine the effectiveness of public health measures in controlling infectious disease outbreaks. The two main measures are isolating symptomatic individuals and tracing and quarantining their contacts. The success of these measures depends on the proportion of transmission occurring before symptoms develop or via asymptomatic infection (denoted as θ), rather than the inherent transmissibility of the pathogen (measured by the basic reproduction number R₀). The study estimates θ and R₀ for four viruses: severe acute respiratory syndrome (SARS), HIV, smallpox, and pandemic influenza. It concludes that SARS and smallpox are easier to control using these measures due to their lower R₀ and θ values. The study emphasizes the importance of estimating θ through contact tracing during an outbreak. It also highlights the impact of delays in isolation and contact tracing on the effectiveness of these measures. The analysis shows that for diseases with high θ values, contact tracing is crucial to counteract delays in isolation. The study provides a framework for predicting the success of public health interventions based on R₀ and θ, and underscores the need for further research into the impact of contact tracing and other targeted control measures on disease transmission dynamics.