1. Core Knowledge:
Think, first, of the "epidemiologic triad" of a disease agent, a susceptible host, plus an environment that brings the host and agent together.
Disease requires all three to arise, and to spread.
(Note, this can apply to infectious and non-infectious diseases)
There are six common ways in which infectious agents spread:
Direct Direct physical contact (body surface to body surface) between infected or colonized individual and susceptible host.
Examples of transmission: Shaking hands; kissing; coitus.
Examples of infections: Common cold; sexually transmitted diseases.
Precautions: Hand hygiene; masks; condoms.
Indirect Infectious agent deposited onto an object or surface (fomite) and survives long enough to transfer to another person who subsequently touches the object.
Examples of transmission: Not washing your hands between patients; contaminated instruments
Examples of infections: RSV; Norwalk; rhinovirus; perhaps influenza.
Precautions: Sterilizing instruments; disinfect surfaces and toys in school.
Droplet Contact, but transmission is through the air. Droplets are relatively large (>5 µm) and projected up to about one metre.
Examples of transmission: Sneezing; coughing, or (in health care) during suctioning
Examples of infections: Meningococcus; pertussis; influenza (maybe: there is some debate); respiratory viruses.
Precautions: Masks; cover mouth; move school desks more than a metre apart; side-step that cougher! Special ventilation not required as aerosolization does not occur.
Note for Nerds: sometimes you will see droplet transmission classified in a separate category intermediate between contact and non-contact. This makes good sense, but do not lose sleep over such debates.
Non-contact Airborne Transmission via aerosols (airborne particles <5µm) that contain organisms in droplet nuclei or in dusts. Can be spread via ventilation systems.
Examples of transmission: Via ventilation system in a hospital;
Examples of infections: TB; varicella; measles; chickenpox; smallpox (and maybe influenza: controversial, as more likely via droplets).
Precautions: Masks; negative pressure rooms in hospitals
Vehicle A single contaminated source spreads the infection (or poison) to multiple hosts. This can be a common source or a point source.
Examples of transmission - Point source: Food-borne outbreak from infected batch of food; cases typically cluster around the site (such as a restaurant); IV fluid; medical equipment.
Common source: The cyanide poisoning of Tylenol in Chicago in 1982. Cases may be widely dispersed due to transport & distribution of the vehicle (here, a medication)
Precautions: Normal safety and disinfection standards. Package medicines in tamper-proof containers.
Vector borne Transmission by insect or animal vectors.
Example of infections: Mosquitoes and malaria.
Precautions: Protect via barriers (window screens, bed nets); insect sprays; culling animals.
2. Nice to Know:
Terms describing spread of an infection
To survive and spread, an infectious agent has first to enter the host, then reproduce itself, and then find a way to spread to other hosts. These various stages have different names, but unfortunately the terms are used somewhat differently by different authorities. Here is a diagram that tries to clarify the stages in the progression of an infectious disease:
3. Additional Information:
Linking the Mode of Transmission to Virulence
Virulence is the harmfulness of an infection, the degree to which it destroys its host. Virulence varies with the mode of transmission:
- Microbes whose transmission depends on close proximity between the host and a susceptible person do not benefit from being highly virulent, for they require the host to be fit enough to move around and transport them to new hosts. A good example is the common cold which causes us to sneeze and spread it via droplets, but does not incapacitate us enough to limit its spread by forcing us to remain isolated in bed.
- However, pathogens that use an intermediate vector for transmission (mosquito, fly or flea) may still survive and spread even if the host is incapacitated. If you are laid up, sweating in bed with malaria, you open the window, throw off the bedcovers and so make it easier for new mosquitoes to bite you and ingest malaria parasites. Indeed, the heavier the parasite load in your body, the more the mosquito will ingest, increasing the chances of spreading the infection when the mosquito bites someone else. Here, high virulence is compatible with transmission and thereby with survival of the pathogen. Similarly, cholera benefits from evolving towards high virulence: the more the bacterium reproduces, the more diarrhoea the infected person excretes, increasing the chances that the bacterium will spread.
- Hence, if the microbe has a good delivery system that does not involve mobility of the host, it benefits from evolving towards high virulence. In general, a disease that spreads via attendants (e.g., nurses or physicians who carry the pathogen from patient to patient but are not themselves infected) tends towards high virulence. For a pathogen to spread via human to human touch, it has to be more virulent so that a smaller infectious dose is needed. A small dose of a less virulent one won't transmit the infection as successfully.
- Pathogens that are durable in the environment (such as anthrax) can also afford to become virulent because they can survive while waiting a long time to be picked up by a susceptible host. Their infectivity increases the chances they infect the new host and, being long-lived, the pathogen does not need the host to survive long for transmission. [A possible exception is the norvirus, which stays viable on hard surfaces for a couple of days and yet remains mild; perhaps the couple of days is not long enough].
- Hence, high virulence is characteristic of vector-borne, waterborne, and attendant-borne transmission, and of durability in the external environment. By contrast, STIs are less virulent as they must remain viable in the host for relatively long periods (at least as long as the interval to a new sexual encounter). Syphilis was known as the great pox; some say it had to grow weaker as its victims were dying or not having sexual contact due to severity of their illness.
Ewald has argued that we might exploit this phenomenon and actively encourage organisms to evolve less virulent forms, rather than escalating the fight by producing increasingly strong antibiotics. Protecting the water supply is good, as fewer people get sick from dirty water; but an additional benefit might be that the prevalent strain of cholera will become less virulent because transmission will depend on people remaining mobile for it to spread. [PW Ewald. Evolution of infectious disease. New York: Oxford University Press, 1994.]
So, this helps explain why infections cause irritating symptoms:
Paul Ewald has a video talk on pathogenicity
- Consider a sneeze: it helps you get rid of the virus in your upper respiratory tract. But from the viewpoint of the virus, it also gives it a free ride to your family members or friends. It is a symptom with a purpose, and may have evolved that way as the transmission helps ensure the virus’s survival.
- Likewise, the urge to scratch that follows infections such as pinworms aids transfer to another host. The female pinworm moves out of the infected child's lower intestinal tract and deposits her eggs near the child’s anus, along with an allergen that causes itching. The child scratches, the eggs get under the fingernails, and are then transferred to a toy or another person. When that person touches their mouth or chews on the toy, some of the eggs may be transferred.
- Diarrhoea is a prominent symptom of cholera and many other intestinal infections. The diarrhoea serves as a transmission channel that gets the disease back into the water system and thence to another host.
- The pathogen faces a trade-off between benefits due to increased exploitation of the host, and the costs that result from the disease and resulting incapacity that reduces the host's infectious contact with other susceptible people.
Updated October 14, 2014