What is the reason WHO says the coronavirus “most likely” originated in animals. (Alternatively:) Why isn’t it just as likely it originated in humans? MBG02 (talk) 23:18, 10 February 2021 (UTC)[reply]

This paper says that a very similar virus has been found in bats, but is different enough that COVID would have split off from it 50 years ago. This suggests an origin in animals. --142.112.149.107 (talk) 03:42, 11 February 2021 (UTC)[reply]

Many human coronaviruses have their origin in bats, but rats are culprits too. See diagram and Coronavirus#Origin.--Shantavira|^{feed me} 09:30, 11 February 2021 (UTC)[reply]

See also Origin of SARS-CoV-2 - 26 March 2020 from the WHO, which also points to the initial outbreak in the infamous Huanan Seafood Wholesale Market where all kinds of wild and domestic creatures were kept and slaughtered in appalling sanitary conditions. Alansplodge (talk) 11:52, 11 February 2021 (UTC)[reply]

Bats are special in that they aggregate in large numbers for long periods in confined spaces, while having an immune system that protects them more than is usual in other mammal orders.^[1][2] (These two may be related.) In any case, that makes them a perfect breeding ground for developing mutations in infectious pathogens of respiratory illnesses, such as several coronaviruses.  --Lambiam 13:14, 11 February 2021 (UTC)[reply]

It's also important to note that nearly every harmful infectious disease to humans is a zoonotic disease, which is to say it was in animals and made the jump to humans at some point in history. The general idea is that it is not really great from an evolutionary standpoint for a virus or bacterium to quickly kill its host. It stands a better chance of survival and a better chance of passing on its genes if it is only mildly symptomatic at best. It only kills humans because it wasn't really evolved to infect humans, it sort of accidentally ended up in humans, and now it's making a mess of things. This video by CGP Grey is about 5 years old, and from a different context, but it does a really good job explaining the animal origins of many human diseases. --Jayron32 00:27, 12 February 2021 (UTC)[reply]

On the other hand, a respiratory virus that doesn't even cause its host to sneeze or cough will have a hard time spreading to new hosts. For the virus it'd be ideal if every host begins to sneeze and cough and nobody dies, but as there are differences between people's immune systems that doesn't happen. So the best thing that can happen for the virus is that many people begin to sneeze and cough, many don't notice a thing and some die. PiusImpavidus (talk) 09:29, 12 February 2021 (UTC)[reply]

Not necessarily. When a human, or even an animal, feels ill, they tend to isolate and separate themselves. As we have seen from Covid-19, respiratory viruses spread just fine asymptomatically. There are many of viruses you are likely carrying now asymptomatically, causing you basically no symptoms, and getting passed along in your normal breath. A classic example is Newcastle disease, which is almost entirely non-virulent in humans. People can carry it, pass it around, and most never get a single symptom, and those that do get symptoms that can easily be confused with a common cold. You could have it right now and you'd never know. However, it is basically Chicken Covid, having jumped from Humans to Chickens at some point in the past causing an avian pandemic in the early 20th century. No one knows how long it was present in humans, even today identifying a non-virulent virus is basically impossible; without symptoms there's not a huge impetus to even check for a virus's existence. --Jayron32 19:08, 15 February 2021 (UTC)[reply]