TITLE: How long does it take for every node of a graph to become infected?
QUESTION [7 upvotes]: Consider the following stochastic process. 
We begin with an undirected graph on $n$ vertices, exactly one of which is ''infected.'' Now at every time step, each infected node infects one of its non-infected neighbors uniformly at random. For example, if a given node has $3$ neighbors that are infected and $5$ that are not, then it infects exactly one of those latter $5$, each of which has probability $1/5$ of being the one chosen. 
Now its clear that after $n-1$ steps every node becomes infected, since at every step at least one non-infected node becomes infected. However, I suspect that the infection actually spreads faster, because as the number of infected nodes grows, more and more non-infected nodes become infected at every stage. 
My question: Is it actually true that every node is infected after $O(D)$ steps, where $D$ is the diameter of the graph?

REPLY [7 votes]: Consider a starlike graph of n+1 nodes in which every node except node A is adjacent to A and only to A.
If A is infected, exactly one of its neighbors becomes infected at each step until all are.  So the number of steps until every node is infected is n, not O(D) since D = 2.