The following question is about SIR (susceptible, infected, recovered) models. Remember, in SIR models you can plot the number of susceptible (S), infected (), and recovered (R) individuals on the y-axis as a function of time (on the x-axis), resulting in a curve for each of the three groups. In your reading, a more realistic model of measles outbreaks is presented. Unlike the simple SIR model discussed earlier in the module, this model includes births, and it predicts that measles outbreaks should cycle over time. How do births affect the disease dynamics?
A) Births increase the number of recovered (resistant) individuals in the population
B) Births increase the number of susceptible individuals in the population.
C) Births increase the probability that infected people transmit the disease.
D) Births themselves occur in cycles, decreasing herd immunity in pulses,