For this model you have the following three equations.

$\displaystyle dS/dt=-BSI$

$\displaystyle dI/dt=BSI-vI$

$\displaystyle dR/dt=vI$

S` is the number of Susceptibles a time t

I` is the number of Infectives at time t

R` is the number who have recovered at time t

B is the disease transmission rate

v is the recovery rate

My question is, if the population is to be assumed constant, why can you neglect the third equation?