Once a disease is established and becomes an epidemic, two key terms help to explain its scale and continued growth, particularly for longer-running epidemics: the prevalence and incidence. The prevalence is the total number of new and existing disease cases within a population over a given time period, while the incidence is the number of new disease cases reported over a period of time. Both terms help to quantify a disease.
When handling outbreaks, the time period that the disease incidence is calculated for is smaller – often days, weeks or months – to identify how the outbreak is spreading and expanding and the level of resources needed to control it, such as vaccine or treatment supplies.
When controlling an infectious disease, the first steps involve isolation and quarantine of cases to limit the infection’s ability to spread, contact tracing of people who may have been exposed to the infection through contact with known cases, and basic hygiene measures and treatments, if available, to further curtail the spread. But an ideal control strategy would also include the availability and use of a vaccine against the disease.
For diseases that are spread from person to person and have effective vaccines to hand, a key part of control efforts at the population level involves mass vaccination in order to reach the herd immunity threshold. This is a level at which a disease pathogen can no longer spread throughout a population – a herd of people – because enough of them are protected by the vaccine to block the pathogen transmitting to the number of people it needs to reach in order to survive. Its chain of transmission is blocked.
It could be argued that the most important factor we need to understand about a disease in order to control it, and end it, is how it spreads: its transmission route. This determines how and where an outbreak occurs and how it then rampages through a community as it grows. Its journey is by no means straightforward; with some diseases having multiple routes of transmission to ensure the infection spreads, and therefore survives.
There are six main ways in which infectious diseases transmit. These are through direct contact with a person’s bodily fluids; through the air; via water; via oral contact with contaminated faces, known as faecal-oral; via a vector; and from mother to child, either in the womb, during childbirth or through breastfeeding.
The infections that spread through the air, such as measles and influenza, are typically the most infectious, though blood-borne infections, such as HIV, can be directly transferred from one person’s blood to another, meaning that though the likeliness of this happening is smaller than airborne spread, the chances of transmission are higher when it does occur.
The most complex transmission, perhaps, is vector-borne, as it involves a pathogen having life cycles within both humans and the vector in order to transfer from one to the other. Transmission occurs through bodily fluids such as blood, open sores, saliva or fluids involved in sex, like semen and vaginal fluids. Microorganisms that spread in this way are usually unable to survive for long once outside of a host body, hence the need to transmit directly. Diseases that fit into this group include sexually transmitted diseases such as gonorrhea and syphilis, Ebola (through all bodily fluids), chickenpox (through open sores), herpes (through open sores) and conjunctivitis (through discharge from the eye). Some diseases have multiple routes of direct transmission, such as HIV, which can spread through blood and semen, vaginal fluids and breast milk.
Many diseases are transmitted through the air, including influenza, tuberculosis, whooping cough (also known as pertussis), and the common cold. Transmission between two people can happen in one of two ways, depending on the disease: an infected person coughs up or sneezes droplets that carry the pathogens short distances to then land on the eyes, ears or nose of a susceptible person (this happens, for example, with meningitis; or the infected person creates a spray that contains the pathogen as they cough, sneeze, talk or breathe (this happens, for example, with measles and tuberculosis, enabling the germs to spread over greater distances and to be inhaled by a susceptible person. The measles virus can persist in the air for up to two hours.
The containment of airborne diseases and many, but not all, diseases transmitted by direct contact requires the immediate quarantine of cases and tracing of people who have been in contact with cases, as well as good hygiene practices and, in the case of airborne diseases, ventilation.
Waterborne diseases are spread through contact with contaminated water, usually by drinking it, where safe drinking water sources are unavailable. Many infections spread in this way overlap as faecal-oral transmission, where an infection is present in a person’s faces and enters water sources through, for example, poor sanitation or sewage infrastructure, or by such infrastructure being overwhelmed by natural disasters and extreme weather, such as flooding.
Source – Outbreaks and Epidemics: Battling Infection in the Modern World by Meera Senthilingam, Brian Clegg (Series Editor)
Goodreads – https://www.goodreads.com/book/show/44784435-outbreaks-and-epidemics
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