An epidemic is the rapid spread of an infectious disease, usually in a particular area over a short period of time. Epidemiologists look for these additional key features
- a high number of infections in relation to the expected number. Endemic diseases (like influenza) return every year but usually at a low and predictable rate.
- spread accelerated by person-to-person transmission
- a rapidly increasing morbidity rate (proportion of the population with disease)
- a population that extends beyond shared accommodation (not a cruise ship, for example, where the world outbreak would be used.)
For endemic diseases an epidemic can be clearly marked on a statistical chart - with a sudden rising curve. Here the data from hospital visits in the US suggests a possible influenza epidemic in the winter of 2007/8
|(credit: modification of work by Centers for Disease Control and Prevention from 'The language of epidemiology')|
With a new pathogen, it is more difficult to identify the start of an epidemic. First is necessary to establish the nature of the disease. Key questions include:
What symptoms do the infected present?
How does it spread?
What characteristics does it share with other known pathogens
What is an outbreak?
The word outbreak is almost synonymous with epidemic in this context - but generally refers to specific location.
In December 2019, hospital doctors in Wuhan began reporting a cluster of patients with "pneumonia of unclear cause". On January 9 China confirmed the outbreak of a new coronavirus and published its genome. This established a link to Sars and Mers viruses.
Scientists were now able to develop tests for the virus but they were now dealing with a rapidly escalating epidemic in Wuhan. The infection rate had shot up across the city and throughout Hubei Province. And new cases were beginning reported in other locations:
|These were the reported cases on Jan 24 2020. The Chinese authorities have now conceded that the true infection rate was least double that indicated.|
What is a pandemic?
A pandemic is the word used when an epidemic spreads to multiple regions or worldwide. This has been a controversial topic in this crisis. The WHO (World Health Organisation) did not declare Covid-19 a pandemic until March 12, 2020. By this point, there were major outbreaks is multiple countries.
A pandemic often involves a new pathogen or pathogen strain. This is because a disease will spread more quickly when it is attacking a host with little or no immunity to it. Over time the virulence of a disease will decrease as a result of what has become known as 'herd immunity'.
The most effective way to achieve this immunity is through the widespread application of a vaccine. That is why so much intensive scientific effort is being applied to this task.