Fact: The Ebola virus has a high pathogenicity but a low infection rate!
Ebola virus disease (commonly just shortened to Ebola) is a dangerous disease spread by contact with body fluids from an infected human or animal. It is a type of viral hemorrhagic fever (VHFs), which in turn are fevers that include increased risk of or susceptibility to bleeding. Some VHFs are not particularly deadly such as Nephropathia epidemica, but Ebola is well known for the potential to prove fatal. Fortunately, while it is true that Ebola is very dangerous to those who become infected, it is a relatively easy disease to prevent from spreading over a population.
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A Scary Disease
Ebola is a zoonotic virus (meaning it is a disease found in animals which can be transmitted to humans) which is potentially very deadly. It is believed to be transmitted by contact with certain species of fruit bats within central and sub-Saharan Africa. Beyond these bats, humans and great apes such as gorillas can also become infected and transmit the disease. First discovered in 1976 people infected with Ebola suffered high temperatures of about 39°C, vomiting blood diarrhea with blood, abdominal pain, collapsing, and rapid “evolution death” after about 3 days. Since the 1970s infrequent outbreaks have occurred in very localized areas of Africa. In 2014 a large outbreak of Ebola in western Africa brought worldwide attention to the disease once again. As of October 2015 the outbreak made ill more than 28,000 of which 11,299 have died.
Virulence and Mortality
When measuring how deadly a disease is epidemiologists use several different measurements to help explain how dangerous a disease can be. Virulence refers both to how dangerous a disease is once it has caused infection (case fatality rates) as well as how easily it can infect an otherwise healthy individual (infectivity).
Case fatality rates (CFR) compare the number of patients that become infected with the disease, compared to the number of those patients that died. For example bubonic plague, which if left untreated kills 6 out of 10 of those infected has a CRF of 60%. Here is a list of the CFR of several well known diseases.
|Spanish (1918) flu|
There have been several different outbreaks of Ebola, with some outbreaks having a CFR of around 30%, and others having a score as high as 90%. According to the Centers for Disease Control (CDC) the average CFR for all types of Ebola is approximately 50%.
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Infectivity, transmissibility and good news
The other important measurement of infectious diseases is how easily the disease can be passed from person to person. This becomes a major factor in determining how likely a disease is to cause a small outbreak, or even a large epidemic. Vertical transmission of a disease occurs when a parent passes on an illness to a child born to that father or mother, and this aspect of the disease is called transmissibility. Other infections passed from person to person are called horizontal transmission, and for a given disease this is called infectivity.
Both the transmissibility and infectivity contribute to the Basic Reproduction Number (BRN) which estimates the number of people that will likely be infected by a single sick individual over the time they are contagious. The symbol used is R0 and often both the ability to transmit from person to person and the ability to cause serious illness go together. The R0 for Influenza (the flu) is about 2.5, whereas the R0 for Small pox is about 6.
Ebola is a dangerous disease, but because it runs its course so quickly (typically 1-2 weeks) patients that are ill are infectious for a relatively short length of time. This coupled with the fact that it can only be transmitted through ‘close’ contact with a sick person contributes to a low R0 of 1.5 for Ebola. Thus although potentially deadly, Ebola is relatively easy to contain with the proper medical intervention.
To learn more about the 2014 West African Outbreak visit:
- Centers for Disease Control and Prevention: 2014 Ebola Outbreak in West Africa