Rising travel, migration, and worldwide trade are stretching the original limits of these vectors and the diseases they carry, thereby endangering naive populations. Transmission depends on the frequency of the insect vector biting humans. Human-biting rates are an important component of vectorial capability since insects are more likely to spread human infections. Though host-feeding preference fluctuates with the duration of the insect reproductive cycle and energy reserves available, it largely determines these rates and hence the probability of transmission. Affecting our capacity to control insect vectors is the adaptability of Aedes mosquitoes to novel habitats, especially in metropolitan areas where high population concentrations of both humans and mosquitoes expose many more people to the dangers of diseases. Most insects have extensive and varied microbiomes that control important physiological processes like metabolism, reproduction, lifespan, immunity, and play critical roles in the supply of nutrients either lacking or have insufficient diet. In blood-feeding insects, consumption of a blood meal causes a notable growth of the midgut microbiota; moreover, microorganism–pathogen interactions can influence the development of an infection when the blood is contaminated with pathogens.