Poisson Distribution (Influenza)

The number $X$ of people in a certain town who get the influenza follows a

Poisson distribution. The proportion of people who did not get the flu is
$0.01$. Find the probability mass function of $X$.

Ok so the probability mass function formula is $e^{-\lambda}\frac{\lambda^k}{k!}$. I saw the solution but I do not understand how this was solved?