Microorganisms can sometimes jump from animals into humans, leading to dangerous and often fatal diseases. The emergence and rapid spreading of SARS-CoV-2, the virus responsible for COVID-19, suspected to have come from bats, has brought the focus back on these so-called zoonotic diseases, but this new coronavirus is not an exception.
A 2008 study in Nature, updated in 2012, produced a global map of human-animal diseases estimating that some 60 percent of all human diseases and 75 percent of all emerging infectious diseases are zoonotic. There are many ways diseases can reach us and many animals that act as vectors. In fact, there are very few places on the planet where humans are not getting diseases from animals.
Wild animals, farm animals, pets, and pests can all spread disease. A recent study published in Proceedings of the Royal Society B showed that animals that have adapted to live near or among us share more viruses with us compared to animals that have little contact with humans, identifying domesticated species, primates, and bats as having more zoonotic viruses than other species. In fact, these animals are thought to have been the vector for some of the biggest human outbreaks the world has seen, so COVID-19 is not unique in that respect.
Here are some of the better-known diseases that are believed to have made the jump from animals to humans.
Influenza pandemics are probably the most well-known type of global pandemics and of those, the 1918 Spanish flu pandemic remains a frightening specter of how bleak things can get. Between 1918 and 1920, it infected 500 million people – 28 percent of the world's population at the time, with deaths thought to be between 50-100 million. It was so deadly that by the end of 1918, the average life expectancy in the United States dropped by 12 years. Most influenza strains have a high mortality risk for the elderly or very young, as well as those immunocompromised. The Spanish flu mostly affected young adults.
It is not known exactly where the Spanish flu it came from – the Spanish part of the name is actually a misnomer – with some hypothesis suggesting a British hospital camp in France during World War I, or the US. In 1918, the Spanish press was not censored on reporting the pandemic in the same way newspapers in Germany, the UK, France, and the US were, which gave the impression the situation was much worse, and even originated, there.
Influenza viruses spread through coughs or sneezes, which propel small virus-containing droplets into the air that can be breathed in by others. They can also enter the airways by touching contaminated surfaces and touching your face. It's thought SARS-CoV-2 spreads in this way and is the reason why the COVID-19 prevention campaign has focused on hand hygiene and physical distancing.
The virus behind the Spanish flu is the H1N1 influenza virus, and genetic research conducted on retrieved samples of the Spanish flu virus suggests it jumped into humans from birds, although it may have gone through pigs or other non-human mammals first. Influenza viruses can jump from aquatic birds to other animals thanks to the so-call antigenic shift, and there are three ways that this might happen: directly from bird to human; from bird to human via another mammal such as a pig; and finally, as a virus makes a jump across species it encounters another flu virus in the same host, merging together to create a new strain.
Depending on the exact mechanism this could place the emergence of the H1N1 Spanish flu virus in mammals as early 1882, diversifying into the deadly human version around 1913-1915. Another notorious pandemic also caused by an H1N1 strain was the "swine flu" pandemic in 2009-2010, this one originating from pigs in Mexico. Between 700 million to 1.4 billion people contracted swine flu, but despite killing 150,000 -575,000, it is not considered as deadly as previous pandemics.
The bacterium Yersinia pestis has been so devasting to humanity that the disease it causes, the plague, is now synonymous with scourge and devastation in many languages. And for good reason: hundreds of millions of people have succumbed to it throughout history.
There have been three major plague pandemics. It first spread across the Mediterranean and the Middle East between 541 and 542 CE, resulting in the deaths of up to 100 million people. Yersinia was in fact named for the Byzantine emperor Justinian, who suffered a large number of losses. The second plague pandemic, AKA the Black Death, saw one-third of all Europeans killed during 1347-1353. The third happened in the late 1800s, originating in China and spreading globally thanks to rats on steamboats, resulting in over 10 million people deaths. There haven't been any pandemics since, but people still catch the plague today.
The bacterium is spread by fleas. It usually affects rodents but it can easily jump to humans if bitten by these fleas or by eating certain raw animals afflicted by it. There are some suggestions that the crashing population of gerbils in Asia might have pushed the fleas to spread to humans in the case of the Black Death. Many believe that the classic view that rodents spread the plague throughout Europe doesn't work. Recent models exonerate the rats and point the finger firmly at human fleas and body lice, and the general poor hygienic conditions kept by Europeans for centuries.
The more recent Western African Ebola virus outbreak that took place between 2013 and 2016 had an incredibly high mortality rate. Over 28,000 people contracted the virus, mainly in Guinea, Liberia, and Sierra Leone, and 11,310 died, making the mortality rate almost 40 percent. Human to human transmission happens via contact with infected bodily fluids, and it is possible for the virus to spread sexually even over a year after the person has recovered from the disease.
The Ebolavirus is believed to have come from fruit bats, although researchers have not been able to actually find an infectious Ebolavirus in bats. Another scenario is that humans and other animals such as gorillas and duikers came into contact with fruit or pulp dropped by infected bats. The virus can be contracted by contact with blood, organs or bodily secretions of infected animals, including chimps, monkeys, and porcupines found ill or dead, according to the WHO. The virus enters their system, adapts, jumps to humans and spreads by human-to-human transmission.
The World Health Organization (WHO) and United Nations both received criticism for the slow handling of the epidemic, highlighting systemic failures, some of which have yet to be addressed. The then Director-General of the WHO stated: "[T]he world remains woefully ill-prepared to respond to outbreaks that are both severe and sustained."
G7 leaders were also criticized for their response, with people claiming the seven most industrialized countries in the world have not committed enough to the fight against ebola and future pandemics.
HIV & AIDS Pandemic
The most deadly pandemic of the modern era is caused by the human immunodeficiency viruses, HIV, which leads to the acquired immunodeficiency syndrome (AIDS). Both viruses, HIV-1 and HIV-2 are believed to have made the jump from apes and monkeys respectively into humans during the first half of the 1900s (though there's still many uncertainties), although the disease became more widespread in the 1970s. HIV is spread through humans via infected blood and bodily fluids, or through unprotected sex with someone with the virus but not on effective treatment.
The first clinically observed case of AIDS was reported in 1981 in the United States, although cases have retroactively been documented at least a decade earlier. The pandemic was not officially recognized as such for years by governments who underestimated its magnitude. HIV and AIDS were seen as a disease exclusively affecting mostly marginalized groups in society such as immigrants, drug users, and members of the LGBTQ+ community. As the disease took hold in the 1980s, derogatory terms and jokes about how it spread made by those in government detained the crucial response time to the disease, which has now claimed the lives of 36 million people worldwide. There are currently between 31 and 35 million people living with HIV, roughly 21 million of them in Sub-Saharan Africa.
Despite ongoing research, there is still no cure and there is no vaccine, but thanks to the work of activists and scientists across the world therapies and preventative treatments now exist. People living with HIV now have the same life expectancy as people without the virus, and those on effective treatment whose viral load is undetectable can no longer transmit the virus.
If there's one thing we've learned from all of these outbreaks, COVID-19 included, it's that there is precedence for global pandemics arising from brand new viruses and strains. While we may not be able to predict what will be next, response time is crucial.