Source: CDC and WHO.
The basic reproduction number of a contagious disease (known as R0) is the number of people per infected individual that will be generated over the course of its infectious period in a susceptible population. The higher the value of R0, the higher the risk of an epidemic or a pandemic since each individual can potentially infect a larger number of people with the risk of exponential growth. Contagion potential should not, however, be associated with the lethality of a disease. For instance, influenza in its regular form is highly contagious (R0 between 2 and 4) but has low lethality. On the opposite range, Ebola has a lower contagion (R0 between 1.5 and 2.5) but is much more lethal. The most contagious diseases known, measles and Pertussis (whooping cough), have a very high R0 but are easily preventable through vaccination. For COVID-19, although there are several variants with different contagion levels, its R0 is between 1.4 and 3.9.