I was reading this paper (pages 3 and 4 if you need the context), while I saw this equation on $R_\infty$ (the number of people who finally recovered): $R_\infty = 1 - e^{-\lambda k R_\infty}$. But, they say
In order to have a nonzero solution the following condition must be fulfilled: $$ \left. \dfrac{d(1 - e^{-\lambda k R_\infty})}{dR_\infty} \right|_{R_\infty=0} >1 $$
I don't understand why this condition is needed.