Scientists have only just begun to explore why some viruses disappear while others cause pandemics that last for centuries.
It was the year 1002. In an effort to resist the Viking army’s attack, King Ethelred II of England ordered the arrest and execution of all Vikings in England. This event is known as the St. Brice’s Day massacre.
More than 1,000 years later, archaeologists discovered 37 skeletons believed to belong to some of the victims of the massacre, along with an unexpected secret. Upon analyzing the DNA of these remains, researchers found that one victim not only had been killed but also suffered from smallpox, though not from the strain of the smallpox virus known in recent times.
Humankind later eradicated smallpox through a vaccination program. However, the virus that those who may have been ancient Viking warriors contracted belonged to a different strain, previously unknown, and seemed to have quietly disappeared centuries ago.
If so, then smallpox has effectively been “extinct” twice.
Many viruses that caused diseases in the past have “disappeared.” (Illustration: BBC)
Today, the story of infectious diseases caused by new viruses has become familiar. From close contact with infected animals to viruses “jumping” from one species to another, followed by the emergence of a “patient zero” who contracts the disease first, then spreads the virus globally. But how this story of pandemics ends has only recently attracted attention. Why do some viruses disappear while others do not? What happened?
The “Mysterious Disappearance” of SARS
As the threat posed by these tiny living structures grows, scientists are increasingly eager to find answers. Especially as humanity continues to grapple with the unpredictable course of Covid-19.
One of the most recent viruses to “disappear” is SARS. The world became aware of this virus around February 10, 2003, after the World Health Organization (WHO) office in Beijing received an email describing “a strange infectious disease” that had killed 100 people within a week.
The first SARS cases were recorded in Guangdong, a coastal province in southeastern China, famous for restaurants serving exotic meats. At that time, local wet markets were bustling with civet cats, badgers, ferrets, pigeons, rabbits, pheasants, deer, and snakes.
After two years, the SARS virus infected about 8,096 people and killed 774. The situation could have been much worse because SARS met all the “criteria” for global spread: being an RNA virus, it could mutate rapidly and spread through respiratory droplets, making prevention more difficult. Many experts believed that the SARS outbreak could have caused destruction on par with HIV or the 1918 influenza pandemic, which infected one-third of the world’s population and killed 50 million.
Humans have overcome many pandemics with vaccines. (Illustration: BBC)
However, SARS suddenly “disappeared.” By January 2004, there were only a few cases left worldwide, and by the end of the month, the last naturally occurring case was reported. Although a few months later, an outbreak re-emerged, it quickly faded into obscurity, with the “last patient” seemingly being a 40-year-old man named Liu in Guangzhou, China.
What Happened?
In short, humanity was “lucky.” However, some argue that SARS ended due to both effective contact tracing measures and the peculiar development of the virus.
In the case of Liu Jianlun, a 64-year-old who contracted the virus before the disease was specifically identified, the WHO estimated that about 4,000 cases could be traced back to this man. This suggests that without global efforts to eradicate SARS, the outbreak could have unfolded very differently.
But experts say the case of SARS is, unfortunately, extremely rare. Besides SARS, only two viruses have been intentionally made to “go extinct”: smallpox and rinderpest (which causes cattle plague). Humanity has triumphed over these two viruses with vaccines.
Overall, it is generally very difficult for a virus to simply disappear once it has adapted to its environment.
Pandemics around the world have claimed the lives of tens of millions. (Illustration: BBC)
Scenario 1: Viruses Come and Go
Some viruses that cause pandemics are believed to be unlikely to go extinct because they do not have just one host.
Ebola outbreaks in humans frequently appear and then end. There have been at least 26 Ebola outbreaks across Africa since the disease was first identified in 1976, and that only counts those cases with sufficient numbers to draw the attention of health authorities. As long as bats exist, Ebola does not seem likely to “say goodbye” to humanity anytime soon.
In Guinea, a study showed that different Ebola viruses have jumped from animals to humans approximately 118 times. The genetic diversity of the viruses from these outbreaks indicates a concerning level of back-and-forth transmission.
Currently, the fight against Ebola faces many challenges. Among the various types of Ebola, vaccines exist for only some. Even if humans make efforts to eliminate the virus from their communities, the disease can still circulate in bats and return later.
Eradicating the virus in bats, and broadly in the wild, seems almost impossible.
The case of MERS, the globally “infamous” outbreak in 2012, is similar. The disease is believed to have returned to humans hundreds of times since then.
Meanwhile, SARS has no clear host, according to Stanley Perlman, a microbiologist at the University of Iowa.
For Covid-19, the origin of the disease-causing virus – SARS-CoV-2 – has yet to be confirmed. One of the hypotheses proposed is that the virus jumped from bats to an intermediate animal before infecting humans. But scientists are still unclear whether humans can transmit the virus back to animals, which would make eradication efforts much more challenging.
Some viruses that cause pandemics are believed to be unlikely to go extinct because they do not have just one host. (Illustration: BBC)
Scenario 2: Variants Disappear but Illness Remains
A more complex case being studied is influenza. There are two main types of influenza: A and B. For many years, it was believed that influenza A strains continually evolved to infect humans more effectively. However, the latest scientific research reveals a different reality.
It turns out that every influenza virus that existed in human bodies until about 120 years ago has gone extinct. The strain that caused the 1918 pandemic has also disappeared, as has the strain that caused the 1957 avian flu outbreak, which killed 116,000 people in the U.S., and the circulating flu strain in 2009, before the swine flu outbreak emerged.
Thus, influenza strains tend to continue evolving in various directions, but then most suddenly go extinct. Every few decades, a new flu type emerges to replace the old one, often created from a combination of old flu viruses and new viruses from animals.
Interestingly, in this evolutionary process, instead of adapting to humans over time, it appears that some virus strains – like H1N1 – have quietly accumulated useless or even harmful mutations, leading to “self-destruction.”
Some scientists propose that accelerating the mutation process of viruses could bring certain benefits. This idea has been around for some time as a way to eliminate flu and colds – but it has also recently been suggested as a method to combat Covid-19.
However, regardless of how hard humanity tries, some scientists remain skeptical about the possibility of making a virus disappear permanently.
Ian Lipkin, an epidemiologist from Columbia University, stated: “The term extinction can be misleading. Viruses can exist in many places – they can hide in people, in frozen specimens, they can lurk in wildlife and domesticated animals – it is truly impossible to say whether a virus has gone extinct or not.”
