Infectious diseases may have multiple infectious stages with very different epidemiological attributes,
including infectivity and disease progression. These stages are often
assumed to have exponentially distributed durations in epidemiological models. However,
models that use the exponential distribution assumption (EDA) may generate biased and
even misleading results in some cases. This discrepancy is particularly damaging if the models
are employed to assist policy-makers in disease control and interventions. This paper
studies a mathematical model that includes multiple infectious stages and general distributions
for the stage durations (with the exponential distribution as a special case). Formulas
for the control reproductive number, Rc, and the basic reproductive number, R0, are derived,
which can be conveniently applied to models in which specific stage distributions are
assumed. It is also shown that the disease dynamics are determined by the reproductive
numbers.