The commonest source of infection for humans are humans themselves. The parasite may originate from a patient or a carrier.
A carrier is a person who harbors the pathogenic microorganisms without suffering from ill-effects because of it.
Several type of carriers have been identified. A healthy carrier is one who harbors the pathogen but have never suffered from the disease caused by pathogen, while a convalescent carrier is one who has recovered from the disease and continues to harbors the pathogen in his body.
Depending on the duration of carriage, carriers are classified as temporary and chronic. The temporary carrier sate lasts less then six months, while chronic carriage may las for several years.
The term contact carrier is applied to a person who acquire the pathogen from a patient, while the term paradoxical carrier refers to a carrier who acquires the pathogen from another carrier.
Many pathogens are able to infect both human and animals. Animals may therefore, act as a source of human infection. In some instances, the infection in animals may be asymptomatic. Such animals serve to maintain the parasite in nature and act as the reservoir of human infections. They are therefor called reservoir hosts.
Infectious disease transmitted from animals to human beings are called zoonoses. Zoonotic diseases may be bacterial, viral, protozoal, helminthic or fungal.
Blood sucking insects may transmit pathogens to human beings. The disease so caused are called arthropod-borne diseases. Insects such as mosquitoes, ticks, mites, flies, seas and lice that transmit infections are called vectors.
Transmission may be mechanical (for example, transmission of dysentery or typhoid bacilli by the domestic fly). Such vectors are called mechanical vectors.
Extrinsic incubation period
It is the interval between the time of entry of the pathogen into the vector and vector becoming infective.
Beside acting as a vector some insect may also act as reservoir hosts.
Soil and water
Some pathogens can survive in the soil for very long periods. Spores of tetanus bacilli may remain viable in the soil for several decades and serve as the source of infection.
Fungi (Histoplasma capsulated, Nocardia asteroids) and also parasites such as roundworm and hookworm survive in the soil and cause human infection.
Water may act as the source of infection either due to contamination with pathogenic microorganisms (cholera vibrio, infective hepatitis virus) or due to the presence of aquatic vectors.
Contaminated food may act as a source of infection. The presence of pathogen in food may be die to external contamination or due to pre-existent infection in meat or other animal products (salmonellosis).
Text Book Of Microbiology