Munkres definition says the following:
A subbasis $S$ for a topology on $X$ is a collection of subsets of $X$ whose union equals $X$. The topology generated by the subbasis $S$ is defined to be the collection $\mathcal T$ of all unions of finite intersections of elements of $S$.
Now, a collection of subsets of $X$ whose union equals $X$ could be made up of a collection of disjoint subsets of $X$. In that event, the collection $\mathcal T$ as defined above is an empty set? How does it generate the topology then?