I'm reading a theorem about existence of a simple set.  

The definition of an immune set can be found from [here](https://en.wikipedia.org/wiki/Simple_set)

A set ${\displaystyle I\subseteq \mathbb {N} }$ is called immune if ${\displaystyle I}$ is infinite, but for every index ${\displaystyle e}$, we have ${\displaystyle W\_{e}{\text{ infinite}}\implies W\_{e}\not \subseteq I}$. Or equivalently: there is no infinite subset of ${\displaystyle I}$ that is recursively enumerable.

I am not able to understand this definition, because I couldn't come up with example.