John escaped from a region where an unknown infectious disease was becoming prevalent. He managed to escape from it, but later sadly discovered that such disease was being spread to the whole world. Let I(t) denote the number of infected people in the whole world at time t, where t = 0 corresponds to the time when he left the disease-prevalent region. After some studies, John suggested that the spread of this disease could be modelled by the equation d2I/dt2dI/dt12I=0 Furthermore, John determined that, at t = 0, I = 5 and dI/dt= 6. According to this model of John, in which time period would everyone in the world be infected by this disease? (Assume the world population is always 7,000 million.)

The answer is5<t6 however I'm not sure how this answer is obtained