<P> Vectors may be mechanical or biological . A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner . An example of a mechanical vector is a housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption . The pathogen never enters the body of the fly . In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite . Biological vectors are often responsible for serious blood - borne diseases, such as malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness . Biological vectors are usually, though not exclusively, arthropods, such as mosquitoes, ticks, fleas and lice . Vectors are often required in the life cycle of a pathogen . A common strategy used to control vector borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector . </P> <P> Tracking the transmission of infectious diseases is called disease surveillance . Surveillance of infectious diseases in the public realm traditionally has been the responsibility of public health agencies, either on the (inter) national or a local level . Public health staff rely on health care workers and microbiology laboratories to report cases of reportable diseases to them . The analysis of aggregate data can show the spread of a disease and is at the core of the specialty of epidemiology . To understand the spread of the vast majority of non-notifiable diseases, data either need to be collected in a particular study, or existing data collections can be mined, such as insurance company data or antimicrobial drug sales for example . </P> <P> For diseases transmitted within an institution, such as a hospital, prison, nursing home, boarding school, orphanage, refugee camp etc., infection control specialists are employed, who will review medical records to analyze transmission as part of a hospital epidemiology program, for example . </P> <P> Because these traditional methods are slow, time - consuming, and labor - intensive, proxies of transmission have been sought . One proxy in the case of influenza is tracking of influenza - like illness at certain sentinel sites of health care practitioners within a state, for example . Tools have been developed to help track influenza epidemics by finding patterns in certain web search query activity . It was found that the frequency of influenza - related web searches as a whole rises as the number of people sick with influenza rises . Examining space - time relationships of web queries has been shown to approximate the spread of influenza and dengue . </P>

Which describes a way that pathogens can be spread