Essentially it is saying that every 5 days, the population that is infected will double. How long until it reaches 7,000,000,000.

The equation is , where N_0 is the initial population (1 in this case), r is the rate of which it expands, and t is the time. You want to find when this equals 7 billion, so enter 7 billion on the other side and then solve for the time t. e is a constant, about 2.71.