Final answer:
After transmission, a pathogen must adhere to the host's tissues to colonize, facilitated by structures like pili and corresponding surface molecules.
Step-by-step explanation:
Following transmission to a new host during pathogenesis (the production of the disease process), there must be adherence to the target host tissue in order for colonization to occur; pili and complementary surface molecules may both help. The correct option is a) Adherence. This is part of the second stage of pathogenesis, which involves the pathogenic microbes attaching to the cells of the body using adhesion factors.