Which One Poses the Greatest Threat?
Shannon Plaxton
Introduction
In recent years, viruses have been emerging as if from nowhere. At least, that’s how it seems. Media reports on outbreaks of killer viruses like SARS, swine flu, and West Nile virus have raised international fear. Currently, the virus dominating news headlines is a new strain of bird flu (H7N9) in China. Detected last March, the virus transmits easily from birds to humans, killing about one in five people it infects. “This is an unusually dangerous virus for humans,” a World Health Organization official stated during a press conference held at the end of April in Beijing. “Definitely one of the most lethal influenza viruses that we’ve seen so far.”
Other current fears centre around a novel coronavirus (NCoV), a SARS-like virus that has killed about 20 people since 2012. News organisations have been closely watching the creeping count of cases and deaths. Our fear of viruses is clear: which one has the capacity to become a pandemic we can’t control? In a world of global air travel, a deadly virus could sweep round the earth with frightening speed.
The thought of protein-coated fragments of genetic material entering the body silently and invisibly hijacking our own cells is unnerving. Viruses outnumber all other life forms on earth. Their ubiquity becomes apparent when we look at the numbers: scientists have estimated that there are about 1031 viruses on our planet—10 million times the number of stars in our universe. In fact, there are estimated to be 100 times as many virus particles in the human body as there are human cells.
Defining Danger
The public and public health officials alike fear the emergence of a deadly new virus, but how do we define deadly? Important criteria could include the case fatality rate, the basic reproductive rate (a way to measure the transmission potential), and the incubation time (boxes 1-3). The most feared viruses are the ones that kill most of those infected, like rabies, which has a case fatality rate of 100% when untreated, or the Ebola virus, which has a case fatality rate of up to 90%. Yet a virus that kills its host more quickly than it can spread will die out before infecting a significant proportion of the population. Ebola virus has caused just 1300 deaths to date, while the 1918 Spanish flu (with a case fatality rate of around 2.5%) led to an estimated 50 million deaths worldwide.
Another important determinant of deadliness is the mode of transmission. A perpetual fear is that deadly viruses will mutate to develop person to person transmission, allowing rapid spread (rather than, for example, spreading to humans from an infected animal source). The WHO’s recent admission that the novel coronavirus is probably spreading from person to person made the news for this reason.
In late 2011, two research teams caused international controversy when they genetically altered the H5N1 avian flu virus in their laboratory, making it capable of airborne transmission between mammals (and thus potentially allow for human to human transmission). The lead author of one of the studies, virologist Ron Fouchier, described the altered virus as “probably one of the most dangerous viruses you can make.” The development of the highly contagious virus caused an uproar in the scientific community, leading to an intense debate about whether such results should ever be released. Some researchers supported the work, arguing that these results would help develop countermeasures against a future flu pandemic; others were concerned about biosecurity, warning that this information could be turned into a weapon by a rogue scientist.
What does the future hold for our battle against viruses? Their resilient nature makes eradication seem unlikely, so we will need to keep innovating. To conclude with the ominous words of Nobel prize-winning molecular biologist Joshua Lederberg: “The single biggest threat to man’s continued dominance on the planet is the virus.”
