The association between pests and the spread of human and animal diseases has been established for many years. In the late 1800s biologists found a number of links between many human diseases and insect and rodent carriers of the disease-causing pathogenic organisms. Of the many millions of insect species, only a relatively small number are capable of causing immense damage to humans by their ability to transmit disease. Such insects are termed vectors. The rodents generally associated with the spread of human disease are even fewer, being restricted to rats and mice. In many cases the causative organisms of disease undergo some developmental change within the insect vector, e.g. malaria; in others no change takes place and the insect involved is simply a transport mechanism, albeit a complex one in some cases. This group of insects includes many species of the non-biting flies and some species of cockroach both of which live in close association with humans.