If you’re confused, think about HIV/AIDS. Human-immunodeficiency virus infects people. If left untreated, HIV can lead to autoimmune deficiency syndrome, or AIDS. Some people might get infected with SARS-CoV-2 and not get sick at all. Others will come down with symptoms of the disease, Covid-19. (Yeah, it sounds like the name of a robot raven to us, too.)
Q: Where did SARS-CoV-2 come from, anyway?
The first cases were identified at the tail end of 2019 in Wuhan, the capital city of China’s Hubei province, when hospitals started seeing patients with severe pneumonia. Like the viruses that cause MERS and SARS, the new coronavirus appears to have originated in bats, but it’s not clear how the virus jumped from bats to humans or where the first infections occurred. Often, pathogens journey through an intermediary “animal reservoir”–bats infect the animals, and humans come into contact with some product from that animal. That could be milk or undercooked meat, or even mucus, urine, or feces. For example, MERS moved to humans through camels, and SARS came through civet cats sold at a live animal market in Guangzhou, China.
Scientists don’t know why some coronaviruses have made that jump but others haven’t. It may be that the viruses haven’t made it to animals that humans interact with, or that the viruses don’t have the right spike proteins so they can’t attach to our cells. It’s also possible that these jumps happen more often than anyone realizes, but they don’t cause serious reactions, so no one notices.
Q: How do coronaviruses even work?
Coronaviruses are divided into four groups called genera: alpha, beta, gamma, and delta. These little invaders are zoonotic, meaning they can spread between animals and humans; gamma and delta coronaviruses mostly infect birds, while alpha and beta mostly reside in mammals.
Researchers first isolated human coronaviruses in the 1960s, and for a long time they were considered pretty mild. Mostly, if you got a coronavirus, you’d end up with a cold. But the most famous coronaviruses are the ones that jumped from animals to humans.
Coronaviruses are made up of one strip of RNA, and that genetic material is surrounded by a membrane studded with little spike proteins. (Under a microscope, those proteins stick up in a ring around the top of the virus, giving it its name–“corona” is Latin for “crown.”) When the virus gets into the body, those spike proteins attach to host cells, and the virus injects that RNA into the cell’s nucleus, hijacking the replication machinery there to make more virus. Infection ensues.
The severity of that infection depends on a couple factors. One is what part of the body the virus tends to latch on to. Less serious types of coronavirus, like the ones that cause the common cold, tend to attach to cells higher up in the respiratory tract–places like your nose or throat. But their more gnarly relatives attach in the lungs and bronchial tubes, causing more serious infections. The MERS virus, for example, binds to a protein found in the lower respiratory tract and the gastrointestinal tract, so that, in addition to causing respiratory problems, the virus also often causes kidney failure.
The other thing that contributes to the severity of the infection is the proteins the virus produces. Different genes mean different proteins; more virulent coronaviruses may have spike proteins better at latching onto human cells. Some coronaviruses produce proteins that can fend off the immune system, and when patients have to mount even larger immune responses, they get sicker.
This story was last updated on 3/3/20 2:15pm ET
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