Maybe this is why no step 5. It's related to the Pathogen Pyramid.
Level 1- Exposure
Level-2 Infection
Level 3- Transmission
Level 4- EPIDEMIC SPREAD
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3427559/
"A useful conceptual ramework for thinking about the emergence of novel viruses is the pathogen pyramid [30,31] (figure 3). The pyramid has four levels. Figure 3. The pathogen pyramid (adapted from ). Each level represents a different degree of interaction between pathogens and humans, ranging from exposure through to epidemic spread."