Insects that can transmit diseases to humans are referred to as vectors. Vectors are organisms, such as mosquitoes, ticks, flies, and fleas, that can carry disease-causing pathogens from one host to another.

Option 1 - Carriers: While insects can carry disease-causing pathogens, the term "carriers" is not commonly used to specifically refer to insects that transmit diseases to humans. It is a more general term that can apply to any organism that carries a disease-causing agent.

Option 2 - Reservoirs: Reservoirs are organisms or environmental sources where pathogens can naturally exist and replicate. They can be animals, plants, or the environment itself, but they do not necessarily transmit diseases to humans.

Option 4 - Incubators: Incubators are devices or environments used to provide controlled conditions for the growth and development of organisms. The term does not directly relate to insects transmitting diseases to humans.

Therefore, the correct answer is option 3 - vectors, as it specifically refers to insects that can transmit diseases to humans.