Furthermore, there are many diseases that can persist with R 0 < 1, while diseases with R 0 > 1 can die out, reducing the utility of the concept as a threshold. R 0 is also used as a measure of eradication for a disease that is endemic, but issues such as backward bifurcations, stochastic effects, and networks of spatial spread mean that an invasion threshold does not necessarily coincide with a persistence threshold. This results in a reduction of the usefulness of R 0 . For example, it is possible that a disease can persist in a population when already present but would not be strong enough to invade. Finally, the threshold value that is usually calculated is rarely the average number of secondary infections, diluting the usefulness of this concept even further.