Explain how you and a friend could be exposed to the same microbes and only one of you becomes sick.

One is probably because your friend has a stronger immune system than you and is able t fight the bacteria infection. The other is probably that your friend is also infected but does not show any symptoms of the infection, in what is referred to as latent infection. The bacteria enters a lysogenic cycle  in your friend’s body rather than the more virulent lytic cycle (that happens to you).  

cells spend most of their time in interphase

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.