One number can help explain why measles is so contagious
A disease’s R0 is the average number of people a person can infect in an unvaccinated population
Two ongoing outbreaks have dominated headlines in the past few months. Since the beginning of the year, measles has sickened at least 940 people in 26 U.S. states as of May 24. In Congo, Ebola has racked up 1,920 cases and killed 1,281 people since August 2018.
Those numbers are scary, but another number, or rather a range, illustrates the potential of these diseases to do damage. It’s called the basic reproduction number, or R0 (pronounced “R naught”), a ratio that describes the contagiousness or transmissibility of an infectious disease. If one person is infected in an unvaccinated population, R0 gives an estimate of how many people would get sick from that individual, on average. Researchers can use R0 to estimate things like the size of an outbreak, the infectiousness of an emerging disease or the effectiveness of tactics against a hypothetical bioterrorism threat.