A vector is a transmitter of disease from one animal to another or to man. Most of the causal agents of disease, microorganisms, have no means of locomotion and are dependent upon other agencies for their spread and propagation. Some are adapted for transport by such inanimate agencies as wind and water; others depend upon living vectors. Vectors may serve one of two functions in the propagation of disease: they may disseminate, which means to disperse or spread abroad; or they may transmit, which includes the inoculation of a new host, as well as the transport of inoculum. There are various types of animal vectors: arthropods (mosquitoes, flies, lice, fleas, bedbugs, ticks), rodents and snails, which are responsible for the transmission of a tremendous range of diseases, many of them of considerable importance from a public health point of view.
Most of the vector-borne diseases are in general found in the tropical areas of the world. However, some of them have a distribution extending up to the Arctic. The most important and the most widespread of the vector-borne diseases is malaria, of which there are an estimated 400 million cases. The vector is the Anopheles mosquito, sole carrier of the Plasmodium malaria-causing parasite. Many of the most important communicable diseases, in terms of morbidity and mortality, are vector-borne. Where the methods of combating the vector are still experimental and unproved, little progress seems to have been made: no great success has been achieved against the snail that helps to transmit bilharzia (schistosomiasis, with an estimated 200 million sufferers) or the gnat that transmits river blindness (onchocerciasis, with an estimated 20 million sufferers) or the fly that transmits sleeping sickness (trypanosomiasis, which is a constant threat in tropical Africa). These diseases as well as yaws (frambesia) and a variety of intestinal worms and skin infestations still debilitate workers and cause great suffering and premature death in many developing countries, especially in Africa. The obstacles to progress in this field are in part financial: high costs may be involved since the problems extend far beyond the range of biological and medical research or even of conventional public health. At issue is control over the territory occupied by the vector. What this may mean in terms of logistics and engineering effort is exemplified by the large-scale bush clearing experiments that have been used in campaigns against the shade-loving tsetse fly. Even the conventional public health measures tend to be extremely costly in developing countries in which basis physical infrastructure is weak or absent. To deal effectively with sewage and to ensure the potability of water supplies may require heavy investment and this may be the only certain way of reducing the incidence of many fly-borne and ingested diseases.