epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population." The target population is the population or species in which the pathogen causes disease; it is the population of interest because it has 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 the symptoms of disease when infected by the pathogen, whereas non-reservoirs show symptoms of the disease. The pathogen still feeds, grows, and reproduces inside a 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 develop 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. A "multi-host" organism is capable of having more than one natural reservoir. Types of reservoirs Natural reservoirs can be divided into three main types: human, animal (non-human), and environmental. Human reservoirs Human reservoirs are human beings infected by pathogens that exist on or within the human body. Poliomyelitis and smallpox exist exclusively within a human reservoir. Humans can act as reservoirs for sexually transmitted diseases, measles, mumps, streptococcal infection, various respiratory pathogens, and the smallpox virus. Animal reservoirs Animal (non-human) reservoirs consist of domesticated and wild animals infected by pathogens. 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. 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. Other zoonotic diseases that have been transmitted from animals to humans include: rabies, blastomycosis, psittacosis, trichinosis, cat-scratch disease, histoplasmosis, coccidiomycosis and salmonella. Common animal reservoirs include: bats, rodents, cows, pigs, sheep, swine, rabbits, raccoons, dogs, other mammals. Common animal reservoirs Bats Numerous zoonotic diseases have been traced back to bats. There are a couple theories that