Which term refers to the transmission of microorganisms directly from one individual to another?

Horizontal transmission is the correct choice because it specifically describes the transfer of infectious agents between individuals of the same generation, typically through direct contact or close proximity. This can occur via various means, such as touching, coughing, or sexual contact, where pathogens are passed from an infected person to a susceptible person.

In contrast, vertical transmission refers to the passing of pathogens from parent to offspring, often during pregnancy, childbirth, or breastfeeding. Indirect transmission involves intermediary objects or agents, such as surfaces or droplets, that carry the microorganism to a new host without direct contact. Vector transmission involves a living organism, such as an insect, that transmits pathogens between hosts. Therefore, horizontal transmission is the most accurate term for direct transfer between individuals.