The number of people that one sick person will infect (on average) is called R₀. Here are the maximum R₀ values for a few viruses.

Source: The Lancet (1918 flu, SARS), University of Michigan School of Public Health (COVID-19, ebola, measles), Johns Hopkins University School of Public Health (chickenpox), Proceedings of the National Academy of Sciences (HIV), Tom Wenseleers at the University of Leuven (COVID-19 delta variant), Australian Government Department of Health (mumps)

Credit: Michaeleen Doucleff, Alyson Hurt and Adam Cole/NPR. Icon by Gerard Higgins/The Noun Project.