Monday, March 30, 2020

How does the coronavirus make the leap from one person to another?

So, there's one thing about this coronavirus pandemic that I've been trying to get my head around, and for which I have not seen a good explanation online (or at least not broken down into a logical set of steps that I can understand), namely the physical mechanism for how the virus is transferred from one individual to another. I don't mean at a cellular or biochemical level. I mean at a gross, human activity level.
Bear with me here. I understand that the SARS-CoV-2 virus is generally transmitted through droplets (respiratory droplets and droplet nuclei), and typically not by viral particles that are not atached to water droplets of any kind (airborne transmission), which can therefore stay suspended in the air for much longer. So, yes, I can visualize, at least theoretically, how a person could sneeze and a droplet of saliva or mucus containing the virus could, with a microscopic probability, land directly in another person's mouth, from where it is ingested into the respiratory system, where it then multiplies massively and infects the second person with the virus.
But that is clearly not how it works in practice, most of the time. It can linger in the air for a short time, but the vast, vast majority of infections arise from surface-to-hand-to-mouth/nose contact. So, a virus carrier touches a doorknob, for example, and then another person touches the same doorknob, transfers the virus to his/her hand, which he/she then uses to pick his/her nose or bite his/her nail, and ... voilá! Infection transferred.
My question is how that virus actually gets from the infected individual to the doorknob. I have this cartoon image of swashbuckling virus characters swinging around, leaping and yelling, and grabbing hold of any passing medium of transfer. But how does the virus even get onto the infected person's hand, for one thing? Isn't the virus deep in his/her internal organs, creating havoc and doing damage? Or is the virus also in their sweat or skin oils? Or is it in ALL the cells of their body?
Well, in the absence of other useful resources, I consulted our daughter, in self-isolation in the next city over. She is not a medical doctor, but she is a biology PhD student, and a pretty bright spark, and what she suggested makes pretty good sense to me.
So, no, the virus doesn't really live on your hands or your sweat; it resides in your lungs and throat, where it can inflict maximum damage. It only gets onto an infected person's hands (and later that doorknob) through a sneeze or a cough, or through blowing the nose or, conceivably but less likely, just from talking or breathing messily. (A sneeze typically puts up to 40,000 droplets into the air, coughing 3,000, and talking just 600 per minute). A sneeze or cough can transfers viral particles onto the hands, from where they are transferred to the doorknob. Or an unprotected sneeze or cough can leave a cloud of virus-carrying droplets hanging momentarily in the air, before they fall onto the ground, or onto any surface (or doorknob) that happens to be in the way.
The virus can live without a host for a few days, max (longer on hard, non-porous surfaces than on porous ones). During that time, if people are not dilligent about hand-washing and the sanitizing of surfaces, someone else could then touch the doorknob, smear the droplets onto their own hands, and later transfer it to to their mouth, nose or eyes (how your eyes are connected to your respiratory system is a whole other mystery to me, but apparently there are mucus membranes in the eyes too).
So, yes, that kind of makes sense now. And remember, it's not just a single virus particle in that droplet, it's probably millions. So, what seems like an improbability with an infinitesimal chance of reinfection becomes a bit more plausible. (Incidentally, while we are about it, I also found out that that viruses are not cells, nor are they even composed of cells: they are a set of genes bundled up within a protective fatty lipid shell - a shell that can be easily broken down by soap or alcohol, leaving the proteins inside to degrade and "die").
And one final thing: if Person 2 picks up a glob of virus from that doorknob, and scratches their nose or kneads their forehead, the virus just stays on their skin, and does not infect them. It does not crawl dramatically and inexorably from side of nose to nasal passage. But the problem is that there are more globs of them still on the person's hands, which can then be inserted up a nostril or used to rub tired eyes, at a later time. Or they could scratch their nose again, putting it back onto their hands, etc, etc.
There are just so many permutations and possibilities, which is precisely what makes it so contagious, and such a pain. Hence all the advice, repeated ad nauseam: wash your hands (and face, I now realize) with soap and water, use hand sanitizer whenever soap and water is not available, clean down surfaces and "high-touch areas" regularly with alcohol-based wipes or soap and water, make a conscious effort to not touch your face, avoid unnecessary contact with other people, etc, etc.

No comments:

Post a Comment