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Skip to 0 minutes and 14 seconds JUDITH GLYNN: What is an infectious disease? And how do infections spread? This lecture looks at infection and different modes of transmission. Here’s one definition of infectious or communicable disease– an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly. The important points here is that it’s something that transmits, and this can be direct or indirect.

Skip to 0 minutes and 48 seconds There are several different types of transmission. Infections can transmit person to person. This can require actual contact, skin to skin, for example, staphylococcal skin infections, or from mucosa to mucosa, as occurs in sexually transmitted infections. Body fluids can be an important source of infection as to how things are transmitted. This might be blood or urinal or sweat or other body fluids. And there’s also vertical transmission, mother-to-child transmission as seen, for example, with HIV infection. Infections can also transmit via fomites. These are physical objects that can carry infection. And they often get infected through the body fluids. And they may be things such as handkerchiefs or the bed clothes or the eating utensils.

Skip to 1 minute and 35 seconds And infections can also transmit short distances through droplets. And this is different from airborne infections, where you get an aerosol and infections can travel much larger distances. And this is seen, for example, with measles or tuberculosis. Infections could also transmit indirectly. This could involve a vehicle such as food, as we see with the food poisoning, salmonella or campylobacter or through water, as we see with cholera or hepatitis A. And sometimes they’ll involve a vector. An example would be malaria, where the life cycle goes from the human to the mosquitoes to the human, and the mosquito is the vector responsible for transmitting the parasite onto the human. Some infections have a reservoir.

Skip to 2 minutes and 23 seconds That’s to say, the place where they normally live is outside of man. Their natural habitat could be in an animal or a plant or the environment. If it’s an animal, and the infection transmits to man, it’s known as a zoonosis. And the infection can happen in different ways. It can be direct, for example with rabies. Rabies in the dog gets into the saliva and into humans when they bite. Or it can be indirect, for example with plague. The plague in the rats gets from the fleas and then gets into the humans. Or it could be via water, as in leptospirosis. Rats and dogs get infected with leptospirosis.

Skip to 2 minutes and 59 seconds It gets into their urine, into the water that way, and then they infect the humans. So what about Ebola? The reservoir for Ebola is not man, and it’s thought probably that it’s fruit bats. This is their natural habitat. Fruit bats seem to be able to get infections without getting ill. And its a zoonosis. It can be transmitted from the bats by eating the bats or, of course, bats infect other animals from eating other animals that are infected or contact with these animals. It’s probably also possible to transmit from fruit that bats have half eaten that’s then got fomites from their saliva, and then it gets infections into people.

Skip to 3 minutes and 42 seconds But Ebola can also transmit person to person after that initial infection has happened from the bats or bush meat And this occurs through actual contact– and body fluids are particularly important– from fomites, from physical objects, and probably also over short distances. But importantly, Ebola is not airborne.

What is an infectious disease? Infection and modes of transmission

This lecture describes the main modes of transmission of infections and explains the ways in which Ebola is transmitted.

Person-to-person transmission can occur in several ways. It may require actual contact, of skin, mucosa, or contact with body fluids. Transmission may occur via fomites, physical objects that can carry infection, such as bed clothes. Droplets may carry infections over short distances (< 1 metre). Some infections are airborne as aerosols (fine particles suspended in the air) and can transmit much further.

Some infections transmit indirectly via a vehicle such as food or water, where they can multiply, or involving a vector e.g. a living carrier such as a mosquito that can transmit the infection from an infected to a susceptible individual.

A reservoir is the natural habitat of an infectious agent, and can be an animal or plant or the environment.

Zoonoses are infections that are transmitted from animals to humans.

Ebola is a zoonosis. The reservoir is thought to be fruit bats. Infections pass to humans from contact with fruit bats or from contact with animals eaten as bushmeat that have been infected by the fruit bat. Once in humans, Ebola spreads from person to person via body fluids, fomites and over short distances as droplets. Ebola is not airborne.

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This video is from the free online course:

Ebola in Context: Understanding Transmission, Response and Control

London School of Hygiene & Tropical Medicine