Deadly epidemic viruses once 'disappeared without a trace'

Scientists are just beginning to understand why some viruses disappear, while others cause epidemics that last for centuries.

The year is 1002. In an attempt to resist the attack of the Viking army (who inhabited Scandinavia from Denmark, Norway and Sweden), the English king Ethelred II ordered the capture and death of all. even the Vikings in England. The event became known as the Saint Brice Day massacre.

More than 1,000 years later, archaeologists have discovered 37 sets of remains - believed to belong to several massacre victims, with an unexpected secret. When analyzing the DNA of these remains, the researchers found that one victim was not only killed but also contracted smallpox, but not from the recently known smallpox virus.

Humanity has repelled the smallpox virus later with a program of vaccination. But the virus that could have been the ancient Viking warriors contracted - belongs to a different strain, unknown, and seems to have disappeared silently centuries ago.

If so, smallpox would be 'extinct' twice.

Many viruses that caused disease in the past have "disappeared". (Artwork: BBC)

Today, the story of new viral infectious diseases has become familiar. From close contact with infected animals, to the virus 'jumping' from one species to another, to the first 'patient zero', then spreading the virus around the globe. But the story of how this epidemic ends has only been of recent interest. Why do some viruses disappear and others do not? What happened?

'Mysterious Disappearance' of SARS

As the threat these tiny living structures pose grows larger, scientists are increasingly eager to find answers. Especially when humanity continues to struggle with the unpredictable developments of Covid-19.

One of the most recent 'disappearing' viruses is SARS. The world knew about the existence of this virus as early as February 10, 2003, after the World Health Organization (WHO) office in Beijing received an email describing a "strange infectious disease" that had caused the death of the virus. 100 people died in just one week.

The first cases of SARS were reported in Guangdong, China's southeastern coastal province, known for its restaurants serving exotic meats. At that time, the local fresh market was bustling with raccoons, badgers, civets, pigeons, rabbits, pheasants, deer and snakes.

After two years, the SARS virus sickened about 8,096 people and killed 774. The situation could have been much worse because SARS had enough 'criteria' to spread globally: As an RNA virus, it was able to mutate quickly, spread through respiratory droplets - making prevention becomes more difficult. Many experts believed that the SARS epidemic could cause the same level of destruction as HIV, or the flu epidemic of 1918 - when a third of the world's population became ill and 50 million people died.

Humans have conquered many diseases with vaccines. (Artwork: BBC)

However, SARS suddenly 'disappeared'. By January 2004, there were only a few cases left in the world, and by the end of the month the last naturally occurring case was reported. Although a few months later an outbreak broke out again, all was quickly forgotten and the 'last patient' appeared to be a 40-year-old man surnamed Liu from Guangzhou, China.

What happened?

In short, humanity got 'lucky'. But there is also an opinion that SARS ended because of both effective anti-epidemic tracing measures and the strange development of the virus.

In the case of 64-year-old Liu Jianlun, who contracted the virus before the disease was specifically identified, WHO estimates around 4,000 cases can be traced back to this man. That said, without global efforts to eliminate SARS, the epidemic would have turned out very differently.

But according to experts, cases of SARS, unfortunately, are extremely rare. Apart from SARS, only two viruses have been deliberately 'extinct', smallpox and rinderpest (which causes cholera in cattle). Humanity has conquered these two viruses with vaccines.

The rest, in general, for the virus to disappear on its own is very unlikely, once it has adapted to its surroundings.

Diseases around the world have claimed the lives of tens of millions of people. (Artwork: BBC)

Scenario 1: Virus comes back

Some epidemic viruses are considered unlikely to become extinct because they do not have a single host.

Human Ebola outbreaks frequently come and go. There have been at least 26 Ebola outbreaks across Africa since the disease was discovered in 1976, and that's just counting cases where the number of cases is high enough to draw attention to health authorities. As long as there are bats, Ebola doesn't look like it's going to say goodbye to humanity any time soon.

In Guinea, one study found that different Ebola viruses jumped from animals to humans about 118 times continuously. The viral genotypic diversity of outbreaks indicates an alarming rate of re-infection.

Currently, the fight against Ebola is facing many challenges. Of the many types of Ebola, there are only a few vaccines. And even though humans work to eliminate the virus in their communities, the disease can still circulate in bat hosts and then return.

Eliminating viruses in bats, and more broadly in the wild, is almost impossible.

The case with MERS, the "famous" global epidemic of 2012, is similar. The disease is said 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.

As for Covid-19, the origin of the virus that causes the disease - SARS-CoV-2 - has yet to be confirmed. One of the theories put forward is that the virus spread from bats to an intermediate animal and then to humans. But scientists still don't know if humans can pass it back to animals, which would make epidemics much more difficult to eradicate.

Some epidemic viruses are considered unlikely to become extinct because they do not have a single host. (Artwork: BBC)

Scenario 2: The mutations disappear but the disease stays

A more complex case to be studied is the flu. There are two main types of influenza, influenza A and influenza B. For many years, it is thought that influenza A strains have evolved to be more effective at infecting people. But the latest scientific research shows a different reality.

It turns out that every flu virus that existed in the human body until about 120 years ago is now extinct. The strain that caused the 1918 pandemic has also disappeared, as has the strain that caused the 1957 bird flu, which killed 116,000 people in the US, and the flu that circulated in 2009, before swine flu emerged.

As such, influenza strains that have already formed tend to continue to evolve in different directions - then largely suddenly go extinct. Every few decades, a new type of flu develops instead, and is usually created from a combination of old flu viruses and new viruses from animals.

Interestingly, during this evolution, rather than adapting to humans over time, it seems that some strains of the virus – like H1N1 – have been silently accumulating mutations that are useless or even harmful to humans. itself, should lead to 'self-destruct'.

Some scientists have suggested that speeding up the virus's mutability could be beneficial. The idea has been around for a while as a way to get rid of the flu and colds - but it's also recently been proposed as a way to fight Covid-19.

However, no matter how hard humanity tries, some scientists still doubt the ability of a virus to disappear forever.

'The term extinction can be misleading,' says Ian Lipkin, an epidemiologist from Columbia University. Viruses can be present in many places - they can be lurking in people, stored in freezers, they can be lurking in wildlife and domestic animals - it's not really possible to say a virus has passed. extinct or not'.