Which of the following diseases is not transmitted by insects?

Diphtheria is caused by the bacterium Corynebacterium diphtheriae and is primarily transmitted through respiratory droplets from infected individuals, as well as by contaminated surfaces or objects. Diphtheria does not rely on insect vectors for its transmission, distinguishing it from the other diseases listed.

In contrast, encephalitis, bubonic plague, and tularemia are all transmitted through insect vectors. Encephalitis can be transmitted by mosquitoes, especially in cases caused by West Nile virus or Eastern equine encephalitis. Bubonic plague is famously transmitted by fleas that infect rodents, while tularemia is often spread by ticks and deer flies that carry the causative bacterium Francisella tularensis. This vector-based transmission ties these diseases directly to insects, whereas diphtheria's transmission pathway is entirely distinct and does not involve insects at all.