How Infectious is COVID-19?
Since this is a current topic of discussion, I found a great article in PopularScience about this very topic. A measure used to quantify how a disease spreads through a population is the "basic reproduction number," known as R0 (pronounced "R naught,"). This number is a measure of how many people, on average, an infected person will infect with a disease. While it doesn’t provide information on the death rate or how deadly a disease is, R0 is a measure of how infectious a disease is and how many people it might impact over time.
If R0 is less than one, the disease will typically die out with each infected person having a less than 1 chance of passing it on one additional individual. An R0 larger than 1 means each infected person infects more than one person on average. With each infected person, others can then, in turn, be infected, spreading the disease throughout the population.
See the image below for various diseases. The typical seasonal flu virus has an R0 of around 1.2. This means that for every five infected people, the infection will spread, on average, to six new people, who pass it along to others.