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
![child scratching measles rash](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2019/05/060819_ht_measles_scivis_feat.jpg?fit=860%2C460&ssl=1)
GOING VIRAL Measles is extremely contagious. And one number, or rather a family of numbers, sums up the risk of its spread.
solidcolours/iStock/Getty Images Plus