What R0 and Rt actually measure
R0, the basic reproduction number, is the average number of secondary infections one case produces in a population that is fully susceptible and where no control measures are in place. It is a property of a pathogen in a particular setting, not a fixed universal constant, because it depends on contact patterns and biology together. Rt, the effective or time-varying reproduction number, measures the same average at a specific moment, after immunity, behaviour change, and interventions have reduced the pool of susceptible contacts.
The practical dividing line is one. When Rt is above one each case more than replaces itself and an epidemic grows; when Rt falls below one transmission shrinks. R0 alone cannot tell you whether spread is currently rising, which is why surveillance tracks Rt over time.
Common misreadings to avoid
A high R0 does not by itself imply a high death toll, since severity is separate from transmissibility. Reported R0 ranges also vary widely between studies because they depend on local contact structure and the methods used to estimate them, so a single quoted figure should be treated as an estimate within a range.
This is general educational information about epidemiological modelling, not medical or policy advice.