How contagious are viruses?

Every virus has an R0 factor — an estimate of how many people are infected by one sick person in the absence of interventions like vaccines and infection control. Here are the maximum R0 values for familiar and emerging viruses.

Note: The infectiousness of COVID has varied over time as different variants spread at different rates, but it has generally become more contagious. This is the average rate calculated in 2022.

Source: Clinical Infectious Diseases (COVID-19); New England Journal of Medicine (hantavirus); The Lancet (1918 flu, SARS); University of Michigan School of Public Health (ebola, measles); Journal of Theoretical Biology (chickenpox, mumps); Proceedings of the National Academy of Sciences (HIV)

Credit: Gabrielle Emanuel, Maria Godoy, Alyson Hurt, Carmel Wroth, Michaeleen Doucleff and Adam Cole/NPR. Icon created by Alice Design from The Noun Project