When Using Host-Pathogen Coevolution to Study Patterns of Human Migration

When using host-pathogen coevolution to study patterns of human migration, why do researchers study human-specific pathogens that transmit vertically as opposed to laterally?

A) Vertical transmission (parent to offspring) provides a shared evolutionary history between host species and pathogen; the evolutionary history of the pathogen should reflect the human population it infects. Lateral transmission can be movement to an unrelated individual or an entirely different population or nonhuman reservoir.
B) Vertical transmission (from population to population) provides an evolutionary history between the host species and the pathogen, reflecting a broad range of evolution within the human population. Lateral transmission (parent to offspring) only provides information on a single lineage within the population.
C) Vertical transmission (sibling to sibling) provides a shared evolutionary history between host and pathogen; the evolutionary history of the pathogen should reflect the human population it infects. Lateral transmission can be movement to other subpopulations, distorting the data within the larger population.
D) Vertical transmission (between unrelated individuals) provides a broad evolutionary history of the population due to high rates of transmission of the pathogen within the population. Lateral transmission is movement to an entirely different population or nonhuman reservoir, so it would not provide a history of migratory patterns.