A vector carries the infection from one host to another either as part of the transmission process, such as a mosquito, or it can be mechanical, for example, through the housefly, which inadvertently transmits organisms to the host on its feet and mouth parts. All vectors of importance are either insects, mosquitoes, flies, fleas, lice, etc. or arachnids, ticks and mites. Infection may occur due to the feeding of the arthropod or as a result of its habits. The cycles of transmission are:

• direct insect to human as in malaria;

• insect to animal with humans entering the cycle as an abnormal host (e.g. bubonic plague);

• insect to animal including humans, from whom it is transmitted to other humans by the same or another insect vector (e.g. yellow fever and East African sleeping sickness).

(Snails, especially in descriptions of schis-tosomiasis, are often called vectors, but they do not carry the infection from one host to another and act only as intermediate hosts.)

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