A. Human beings
B. Animals
C. Insects
D. Soil and water
E. Food
A. Human Beings
The commonest source of infection for human beings is human beings themselves. The parasite may originate from a patient or carrier. Humans play a substantial role as microbial reservoirs.
Humans serving as the microbial reservoir
i. Indeed, the passage of a neonate from the sterile environment of the mother’s womb through the birth
canal, which is heavily colonized with various microbial agents, is a primary example of one human
directly acquiring microorganisms from another human serving as the reservoir.
ii. Acquisition of “strep” throat through touching.
iii. Hepatitis by blood transfusions.
iv. Gonorrhoea, syphilis, and (AIDS) acquired immune deficiency syndrome by sexual contact.
v. Tuberculosis by coughing; and the common cold through sneezing.

Carrier
A carrier is a person who harbours the microorganisms without suffering from any ill effect, because of it. There are several types of carriers.
1. Convalescent carrier: A convalescent carrier is an individual who has recovered from the infectious disease but continues to harbour large numbers of the pathogen.
2. Healthy carrier: A healthy carrier is an individual who harbours the pathogen but is not ill.
3. Incubatory carrier: An incubatory carrier is an individual who is incubating the pathogen in large
numbers but is not yet ill.
4. Temporary carriers: Convalescent, healthy, and incubatory carriers may harbour the pathogen for
only a brief period (hours, days, or weeks) and last less than six months and then called casual, acute,
transient or temporary carriers.
5. Chronic carriers: They harbour the pathogen for long periods (months, years, or life).
6. Contact carriers: The term contact carrier is applied to a person who acquires the pathogen from a
patient.
7. Paradoxical carrier: Paradoxical carrier refers to a carrier who acquires the pathogens from another carrier.

Carriers may be classified according to portal of exit of the infectious agent such as urinary carriers, intestinal carriers, respiratory carriers, nasal carriers, etc.

B. Animals
Reservoir Hosts
Many pathogens are capable of causing infections in both human beings and animals. Therefore, animals may act as a source of infection of such organisms. These animals serve to maintain the parasite in nature and act as a reservoir and they are, therefore, called reservoir hosts.
Zoonosis
The diseases and infections, which are transmissible to the man from animals are called zoonosis. Humans contact the pathogens by several mechanisms.
Examples of zoonotic diseases:
Bacterial
Anthrax, brucellosis, Q fever, leptospirosis, bovine tuberculosis, bubonic plague, Salmonella food poisoning.
Viral
Rabies, yellow fever, cowpox, monkeypox
Protozoal
Leishmaniasis, toxoplasmosis, trypanosomiasis, babesiosis.
Helminthic
Echinococcosis, teniasis, trichinellosis
Fungal
Microsporum canis, Trichophyton verrucosum.
C. Insects
Arthropod-borne Diseases
Blood-sucking insects such as mosquitoes, ticks, mites, flies, and lice may transmit pathogens to human beings and diseases so caused are called arthropod-borne diseases.
Vectors
Insects that transmit infections are called vectors. Vectorborne transmission can be of two types either mechanical (external) or biological (internal).
i. Mechanical vector: The disease agent is transmitted mechanically by the arthropod. Carriage is passive, with no growth of the pathogen during transmission.
Examples: Transmission of diarrhoea, dysentery, typhoid, food poisoning and trachoma by the housefly.
ii. Biological vectors: Biological vectors are those in whom the pathogens multiply or undergo developmental changes with or without multiplication. Biological vectors transmit the infection only after the pathogen has multiplied in them sufficiently or has undergone a developmental cycle. The interval
between the time of the entry of the pathogen into the vector and the vector becoming infective is called
the extrinsic incubation period.
Examples: Aedes aegypti mosquito in yellow fever, Anopheles mosquito in malaria.
Reservoir Hosts
Besides acting as vectors, some insects may also act as reservoir hosts (for example, ticks in relapsing fever and spotted fever). Infection is maintained in such insects by transovarial or transstadial passage.

D. Soil and Water
i. Soil
Some pathogens can survive in the soil for long periods.
Examples
a. Spores of tetanus and gas gangrene:  Spores of tetanus and gas gangrene remain viable in the soil for several decades and serve as a source of infection. The human and animal intestine is the normal habitat of these organisms and they enter the soil through their faeces.

b. Fungi and parasites: Fungi (causing mycetoma, sporotrichosis, histoplasmosis) and parasites such as roundworms and hookworms also survive in the soil and cause human infection.

ii. Water
Water may act as the source of infection either due to contamination with pathogenic microorganisms (Shigella, Salmonella, Vibrio cholerae, poliomyelitis virus, hepatitis virus) or due the presence of aquatic vector (cyclops containing larvae of guinea worm infection).

E. Food
Contaminated food may act as a source of infection of organisms causing food poisoning, gastroenteritis, diarrhoea and dysentery. There are two primary types of food-related diseases: foodborne infections and food intoxicants.