Airborne transmission occurs when infectious agents, carried by dust or droplet nuclei suspended in the air, are inhaled into the respiratory system.

• A. When pathogens are carried on water droplets that fall to surfaces.
• B. When transmission occurs via vectors like mosquitoes.
• C. When infectious agents are carried by dust or droplet nuclei and inhaled.
• D. When pathogens replicate inside a host cell.

Answer: (C)

Airborne transmission involves small infectious particles that remain suspended in air and are inhaled into the respiratory system. These particles, such as droplet nuclei from evaporated droplets or dust contaminated with pathogens, can travel over distances and linger in the environment, so inhalation is the route that leads to infection. That aligns with the description: infectious agents carried by dust or droplet nuclei suspended in the air and inhaled.

The other scenarios don’t describe this route. Droplets that are carried on water and fall to surfaces tend to settle quickly and typically spread via contact or surfaces (fomite transmission). Vectors like mosquitoes transmit by carrying pathogens between hosts, which is a different mechanism. Pathogens replicating inside a host cell describe the pathogen lifecycle, not how they move between individuals. Examples of airborne pathogens include tuberculosis, measles, and varicella, which spread through inhaled aerosols.