What is an example of mechanical transmission of disease?

Mechanical transmission of disease refers to the process by which an infectious agent is physically carried from one host to another by a vector, without undergoing any development or multiplication in the vector. In this scenario, pink eye (conjunctivitis) being transmitted by sand gnats illustrates this concept well. The sand gnat serves as a carrier that transfers the infectious agent mechanically - by carrying pathogens on its body or in its saliva, without being part of the pathogen's life cycle itself.

In contrast, the other options describe different modes of transmission. For instance, the flu is primarily spread through respiratory droplets, which involves direct transmission through the air rather than mechanical transfer by a vector. Malaria involves a biological transmission mechanism where the malaria parasite undergoes development within the mosquito, indicating a more complex interaction than mechanical transmission. Similarly, HIV transmission through blood involves direct contact and transfer of the virus rather than mechanical carriage. Therefore, pink eye being transmitted by sand gnats serves as a clear and direct example of mechanical transmission.