Microbiology
Direct horizontal transmission requires a nonliving intermediate such as water or a surface, while indirect horizontal transmission requires direct skin-to-skin contact between infected and susceptible hosts.
Direct horizontal transmission is limited to animal-to-human events (zoonoses) and indirect horizontal transmission is limited to human-to-human contact situations.
Direct horizontal transmission only occurs via airborne fomites that remain suspended for long periods, while indirect horizontal transmission occurs only when pathogens are passed through vertical routes like breastfeeding.
Direct horizontal transmission involves immediate transfer of pathogens between hosts (e.g., droplet spread during close contact or bite), whereas indirect horizontal transmission involves an intermediate vehicle such as fomites, food, water, vectors, or environmental reservoirs.
