Mathematical model for malaria
I am afraid you may have to read your textbook more closely. The question on page 29 states that this equation models the proportion of infected humans. If $I$  $≥1$ that would mean that there are more people are infected than there are people in total - e.g. 3/2 of people are infected - this clearly doesn't make sense. 

Thus $1≥I$ when using this model.