Epidemic vs. endemic, R0 & S

epidemic--> the rate of a disease spreading throu a population is larger than expected. eg, epidemic outspread of plaque in europe, epidemic AIDS in Africa.

Endemic--> the rate of a disease remains the same; no increase nor decrease.
To be endemic, R0 x S=1 (an infected individual can spread to 1 person only on average. R0=1).

R0 is the basic reproductive number, meaning the susciple number of seondary infections an infected individual can spread. If R0>1, the disease will spread in the population and become epidemic. If R0<1, disease will fade out.


S stands for suspecible population, meaning w/o any vaccine.