The disease-causing organisms transferred by the insect may be viruses (termed “arboviruses”, an abbreviation of arthropod-borne viruses), bacteria (including rickettsias), protists, or filarial nematode worms. Replication of these parasites in both vectors and hosts is required and some complex life cycles have developed, notably amongst the protists and filarial nematodes. The presence of a parasite in the vector insect (which can be determined by dissection and microscopy and/or biochemical means) generally appears not to harm the host insect. When the parasite is at an appropriate developmental stage, and following multiplication or replication (amplification and/or concentration in the vector), transmission can occur. Transfer of parasites from vector to host or vice versa takes place when the blood-feeding insect takes a meal from a vertebrate host. The transfer from host to previously uninfected vector is through parasite-infected blood. Transmission to a host by an infected insect usually is by injection along with anticoagulant salivary gland products that keep the wound open during feeding. However, transmission may also be through deposition of infected feces close to the wound site.
In the following survey of major arthropod-borne disease, malaria will be dealt with in some detail. Malaria is the most devastating and debilitating disease in the world, and it illustrates a number of general points concerning medical entomology. This is followed by briefer sections reviewing the range of pathogenic diseases involving insects, arranged by phylogenetic sequence of parasite, from virus to filarial worm.