Content Reviewers:Yifan Xiao, MD
Emerging diseases are illnesses that have increased in incidence during the past 20 years, or are expected to increase in the near future.
Some are zoonotic, meaning they are caused by pathogens that have hopped from infected animals to humans.
A few examples are coronaviruses, a group of RNA viruses that can cause a variety of respiratory illnesses in humans.
Transmission of emerging zoonotic diseases can occur through animal tissues, environmental sources; or animal hosts, either directly or indirectly through another animal, or even an inanimate carrier, like air.
And they can even reemerge through similar mechanisms if the pathogen has a natural reservoir, which can be an animal where it naturally survives and reproduces.
And there are a few factors that help increase their transmission.
Some of the major ones include: increased close contact between an infected animal and humans, which happens during deforestation, climate change, or large-scale farming and food processing; changes in societal norms, like food preferences, or areas to live, and decreases in vaccination rates or sanitation; or increased globalization that spreads local outbreaks rapidly through trade and travel; and even increased populations vulnerable to infections, like older folks and people with underlying conditions which may make them immunosuppressed.
Now, Coronaviruses are a group of enveloped, positive-sense single-strand RNA viruses.
And they were first discovered in the 1930s in domestic poultry that had respiratory, gastrointestinal, liver, and neurologic diseases.
But since then, they have been found in a variety of animals, and seven coronaviruses are currently known to cause disease in humans.
A main way they are transmitted person-to-person is through respiratory droplets, which contain the pathogen and come in direct contact with the eyes, nose, or mouth of another person who comes in close contact; or indirectly make its way there by first contaminating shared surfaces.
Four of the more benign coronaviruses circulating among humans; 229E and OC43, and serotypes NL63 and HUK1, have symptoms typical of the common cold and rarely lead to severe lower respiratory tract infections.
But, three of the seven coronaviruses that affect humans have caused outbreaks of severe and sometimes fatal respiratory infections, including: Severe acute respiratory syndrome coronavirus, or SARS-CoV; Middle East respiratory syndrome coronavirus (MERS-CoV) causing MERS in 2012, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
SARS-CoV caused the SARS outbreak in 2002.
The influenza-like illness can lead to severe respiratory insufficiency and was first reported in the Guangdong province in China.
The virus likely has a reservoir in bats that probably came in close contact with civets, a cat-like mammal.
These infected civets then likely became an intermediary to infect humans after close contact in live-animal markets.