Pathogen Transmission
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Communicable infection
Infection is spread from host to host.
Contagious
Infection is easily transmitted.
Though often used interchangeably, there is a slight, but important, difference:
– For example, the influenza virus is communicable and contagious; if you are in the same classroom as someone carrying the influenza virus, there is a good chance you will "catch" the flu, even if you never touch the infected person.
– HIV is communicable, because it is spread from host to host, but it is not contagious; an uninfected person can share a classroom, or even a hug, with an HIV-positive individual and not become infected.
– Food poisoning is neither communicable nor contagious – you cannot "catch" food poisoning from another human host, you must ingest the contaminated food, yourself, to become infected.
Reservoir
The living or non-living normal residence of an infectious agent.
Zoonotic diseases
Infections caused by pathogens that reside and replicate within non-human animals; for example, certain species of rats carry the hantavirus associated with serious pulmonary disease.
Vectors
The living creatures that transmit a pathogen to humans; for example, ticks are arthropod vectors for bacterial infections such as Lyme disease.
Host
The organism that provides nourishment and/or shelter to the agent.
Vertical transmission:
Occurs when infectious agents are passed from a mother to her offspring during pregnancy, childbirth, or breastfeeding.
Transplacental transmission:
Neonatal sepsis and meningitis are caused by transmission of the bacteria Listeria monocytogenes.
Toxoplasmosis is caused by transmission of the parasite Toxoplasma gondii.
During Childbirth:
Group B streptococcus and Nisseria gonorrhoeae.
Breastfeeding:
Transmit pathogens via the breastmilk itself or from the surface of the breast:
Human Immunodeficiency Virus (HIV) travels in the milk.
Staphylococcus aureus, a bacteria commonly found on the skin, can be transferred during suckling.
Horizontal transmission
Other types of human-human transmission of infectious disease:
Transmission can occur via direct contact, fecal-oral routes, respiratory droplets, or in the blood.
– Examples of diseases transmitted from human to human include influenza, dysentery, and sexually-transmitted infections (STIs, also called sexually transmitted diseases).
Arthropod Vectors
For example, Lyme disease is transmitted by ticks infected with the bacterium Borrelia burgdorferi, and malaria is transmitted by mosquitos infected with Plasmodium parasites.
Other animals
Transmit pathogens to humans through direct contact or via their milk or feces.
– For example, Brucellosis is an infectious disease caused by bacteria that infect sheep, cows, pigs, dogs, and other animals. The Brucella bacteria are most commonly transmitted to humans via consumption of unpasteurized/raw dairy products from infected animals.
Fomites
Inanimate objects that transmit pathogens.
For example, some Staphylococcal skin infections can be traced to contaminated towels or other unclean linens.
Soil
Harbors pathogens.
For example, spores of the bacteria Clostridium tetani are commonly found in soil; upon entry into a flesh wound the spores develop into bacteria that secrete neurotoxin, causing tetanus.
Transmission to Human Hosts