This is one of the best and simplest explanations of Covid-19, its structure, origins, effects etc., and answered so many questions for me that I was confused about. I'm fuzzy on some of the science talked about in the article, but even so, it's been the clearest explanation I’ve read so far
I found this particularly interesting. Respiratory system information:
The structure of the virus provides some clues about its success. In shape, it’s essentially a spiky ball. Those spikes recognize and stick to a protein called ACE2, which is found on the surface of our cells: This is the first step to an infection. The exact contours of SARS-CoV-2’s spikes allow it to stick far more strongly to ACE2 than SARS-classic did, and “it’s likely that this is really crucial for person-to-person transmission,” says Angela Rasmussen of Columbia University. In general terms, the tighter the bond, the less virus required to start an infection.
There’s another important feature. Coronavirus spikes consist of two connected halves, and the spike activates when those halves are separated; only then can the virus enter a host cell. In SARS-classic, this separation happens with some difficulty. But in SARS-CoV-2, the bridge that connects the two halves can be easily cut by an enzyme called furin, which is made by human cells and—crucially—is found across many tissues. “This is probably important for some of the really unusual things we see in this virus,” says Kristian Andersen of Scripps Research Translational Institute.
For example, most respiratory viruses tend to infect either the upper or lower airways. In general, an upper-respiratory infection spreads more easily, but tends to be milder, while a lower-respiratory infection is harder to transmit, but is more severe. SARS-CoV-2 seems to infect both upper and lower airways, perhaps because it can exploit the ubiquitous furin. This double whammy could also conceivably explain why the virus can spread between people before symptoms show up—a trait that has made it so difficult to control. Perhaps it transmits while still confined to the upper airways, before making its way deeper and causing severe symptoms. All of this is plausible but totally hypothetical; the virus was only discovered in January, and most of its biology is still a mystery.
Researchers can, however, offer a preliminary account of what the new coronavirus does to the people it infects. Once in the body, it likely attacks the ACE2-bearing cells that line our airways. Dying cells slough away, filling the airways with junk and carrying the virus deeper into the body, down toward the lungs. As the infection progresses, the lungs clog with dead cells and fluid, making breathing more difficult. (The virus might also be able to infect ACE2-bearing cells in other organs, including the gut and blood vessels.)
The immune system fights back and attacks the virus; this is what causes inflammation and fever. But in extreme cases, the immune system goes berserk, causing more damage than the actual virus. For example, blood vessels might open up to allow defensive cells to reach the site of an infection; that’s great, but if the vessels become too leaky, the lungs fill even more with fluid. These damaging overreactions are called cytokine storms. They were historically responsible for many deaths during the 1918 flu pandemic, H5N1 bird flu outbreaks, and the 2003 SARS outbreak. And they’re probably behind the most severe cases of COVID-19. “These viruses need time to adapt to a human host,” says Akiko Iwasaki of the Yale School of Medicine. “When they’re first trying us out, they don’t know what they’re doing, and they tend to elicit these responses.”
During a cytokine storm, the immune system isn’t just going berserk but is also generally off its game, attacking at will without hitting the right targets. When this happens, people become more susceptible to infectious bacteria. The storms can also affect other organs besides the lungs, especially if people already have chronic diseases. This might explain why some COVID-19 patients end up with complications such as heart problems and secondary infections.
But why do some people with COVID-19 get incredibly sick, while others escape with mild or nonexistent symptoms? Age is a factor. Elderly people are at risk of more severe infections possibly because their immune system can’t mount an effective initial defense, while children are less affected because their immune system is less likely to progress to a cytokine storm. But other factors—a person’s genes, the vagaries of their immune system, the amount of virus they’re exposed to, the other microbes in their bodies—might play a role too. In general, “it’s a mystery why some people have mild disease, even within the same age group,” Iwasaki says.
