1. Definition and Terminology

The great diversity of infectious pathogens, their possible hosts, and the ways in which their hosts respond to infection has resulted in multiple definitions for "natural reservoir", many of which are conflicting or incomplete. In a 2002 conceptual exploration published in the CDC's Emerging Infectious Diseases, the natural reservoir of a given pathogen is defined as "one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population."^[1] The target population is the population or species in which the pathogen causes disease; it is the population of interest because it suffers from disease when infected by the pathogen (for example, humans are the target population in most medical epidemiological studies).

A common criterion in other definitions distinguishes reservoirs from non-reservoirs by the degree to which the infected host shows symptoms of disease. By these definitions, a reservoir is a host that does not experience disease when infected by the pathogen. The pathogen still feeds, grows, and reproduces inside the reservoir host, but otherwise does not significantly affect its health; the relationship between pathogen and reservoir is more or less commensal, whereas in susceptible hosts that do suffer disease caused by the pathogen, the pathogen is considered parasitic.

What further defines a reservoir for a specific pathogen is where it can be maintained and from where it can be transmitted.^[1] A "multi-host" organism is capable of having more than one natural reservoir.^[2]

2. Types of Reservoirs

Natural reservoirs can be divided into three main types: human, animal (non-human), and environmental.^[2]

Human reservoirs are human beings infected by pathogens that exist on or within the human body.^[2] Poliomyelitis and smallpox exist exclusively within a human reservoir.^[3]

Bushmeat being prepared for cooking in Ghana, 2013. Human consumption of animals as bushmeat in equatorial Africa has caused the transmission of diseases, including Ebola, to people.^[4] https://handwiki.org/wiki/index.php?curid=1334754

Animal (non-human) reservoirs consist of domesticated and wild animals infected by pathogens.^[1][2] For example, the bacterium Vibrio cholerae, which causes cholera in humans, has natural reservoirs in copepods, zooplankton, and shellfish. Parasitic blood-flukes of the genus Schistosoma, responsible for schistosomiasis, spend part of their lives inside freshwater snails before completing their life cycles in vertebrate hosts.^[5] Viruses of the taxon Ebolavirus, which causes Ebola virus disease, are thought to have a natural reservoir in bats or other animals exposed to the virus.^[6]

Environmental reservoirs include living and non-living reservoirs that harbor infectious pathogens outside the bodies of animals. These reservoirs may exist on land (plants and soil), in water, or in the air.^[2] Pathogens found in these reservoirs are sometimes free-living. The bacteria Legionella pneumophila, a facultative intracellular parasite which causes Legionnaires' disease, and Vibrio cholerae, which causes cholera, can both exist as free-living parasites in certain water sources as well as in invertebrate animal hosts.^[2][7]

3. Disease Transmission

A disease reservoir acts as a transmission point between a pathogen and a susceptible host.^[8] Transmission can occur directly or indirectly.^[8][9]

3.1. Direct Transmission

Direct transmission can occur from direct contact or direct droplet spread.^[9] Direct contact transmission between two people can happen through the touching of skin, through kissing, and sexual contact.^[8][9] Humans as disease reservoirs can be symptomatic (showing illness) or asymptomatic (not showing illness), act as disease carriers, and often spread illness unknowingly.^[8] Direct droplet spread is due to solid particles or liquid droplet suspended in air for some time.^[9] Droplet spread is considered the transmission of the pathogen to susceptible host within a meter of distance, they can spread from coughing, sneezing, and talking.^[8][9]

• Neisseria gonorrhoeae (Gonorrhea) is transmitted by sexual contact involving the penis, vagina, mouth, and anus through direct contact transmission.^[10]
• Bordetella pertussis (Pertussis) is transmitted by cough from human reservoir to susceptible host through direct droplet spread.^[11]

3.2. Indirect Transmission

Indirect transmission can occur by airborne transmission, by vehicles (including fomites), and by vectors.^[12]

Airborne transmission is different from direct droplet spread as it is defined as disease transmission that takes place over a distance larger than a meter.^[13] Pathogens that can be transmitted through airborne sources are carried by particles such as dust or dried residue (referred to as droplet nuclei).^[12][13]

Vehicles such as food, water, blood and fomites can act as passive transmission points between pathogens and susceptible hosts.^{[14] [15]} Fomites are inanimate objects (doorknobs, medical equipment, etc.) that become contaminated by a reservoir source or someone/something that is a carrier.^[14] A vehicle, like a reservoir, may also be a favorable environment for the growth of an infectious agent, as coming into contact with a vehicle leads to its transmission.^[12]

Vector transmission occurs most often from insect bites from mosquitoes, flies, fleas, and ticks. There are two sub-categories of vectors: mechanical (an insect transmits the pathogen to a host without the insect itself being affected) and biological (reproduction of the pathogen occurs within the vector before the pathogen is transmitted to a host).^[16] To give a few examples, Morbillivirus (measles) is transmitted from an infected human host to a susceptible host as they are transmitted by respiration through airborne transmission.^[17] Campylobacter (campylobacteriosis) is a common bacterial infection that is spread from human or non-human reservoirs by vehicles such as contaminated food and water.^[18] Plasmodium falciparum (malaria) can be transmitted from an infected mosquito, an animal (non-human) reservoir, to human host by biological vector transmission.^[16]