# How does virus infection work?

• Vrbic
In summary, the probability of becoming infected when receiving a tiny amount of a virus on any body part is very high.

#### Vrbic

TL;DR Summary
What is the probability that a healthy person with fully functional immunity will become infected when he receives a tiny amount of a) hands, b) eyes, c) lungs?
What is the probability that a healthy person with fully functional immunity will become infected when he receives a tiny amount of some (flu kind) virus on a) hands, b) eyes, c) lungs?
I don't know how to define a small dose of a virus, but I guess if only one virus gets in my lungs, my immune system can handle it. It is so?
It's probably also dependent on the type of virus. How about ordinary flu and sars-cov2.
Thank you for reactions.

Vrbic said:
I don't know how to define a small dose of a virus, but I guess if only one virus gets in my lungs, my immune system can handle it. It is so?

I'm not an expert in this, but I think the answer is that this is wrong. A single virus particle is enough to start the infection. It infects a single cell in your body, which then produces many more viruses, so the number of viruses in your body grows exponentially. Your immune system does not respond immediately to a virus it has never seen before, so it takes time (several days to a week or so) to ramp up an immune response.

Vrbic said:
Summary:: What is the probability that a healthy person with fully functional immunity will become infected when he receives a tiny amount of a) hands, b) eyes, c) lungs?

It depends on the virus and where the viral particles land.

For norovirus it is thought that about 10 viral particles are enough to cause infection.
https://www.sciencedirect.com/topics/neuroscience/norovirus

For SARS-CoV-2, @Ygggdrasil and @Astronuc posted a paper in another thread with an indirect estimate of how much is needed is needed to be infectious. They attempted to isolate "live" virus, ie. they took samples from patients to infect cells suspended in a medium, and checked to see whether the cells became infected. They were able to estimate that the cells suspended in a medium did not become infected unless there was more than 10,000 viral copies per ml of the patient sample. See Fig 2g of https://www.nature.com/articles/s41586-020-2196-x.

Googling suggests this paper that gives decent evidence that under certain conditions a single virus is enough to cause infection.
https://www.sciencedaily.com/releases/2009/03/090313150254.htm
https://royalsocietypublishing.org/doi/10.1098/rspb.2009.0064

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Phil Core, Vrbic and TeethWhitener
Yes, the type of virus is all important and then there is the issue of how the person is exposed to the virus, there are lots of barriers in place. The virus then has to somehow navigate through the host, which is generally a very hostile environment and which contains a variety of specific and non specific chemical agents that can be very damaging to the virus.

The presence of high levels of specific antibodies makes the possibility of establishing an infection very remote indeed. It must find a cell to which it can gain entry, usually this means the cell has to have specific receptors on its surface that the virus can exploit, it must then co-opt the cells machinery to be transported to the nucleus, while avoiding the inbuilt checking mechanisms. It has to get through another barrier protecting the nucleus and inject its own genetic material.

Cells do have their own quality control checks, and if the viruses are detected the cell can start to express specific markers on its surface that act as red flags to the immune system. If these cells are detected the cell will be destroyed along with its pathogens, the virus has to survive long enough for the new genetic program to produce millions of new virus particles which will be released to infect more cells, this process usually kills the cell.

All the time this is happening the bodies defences become increasingly aware that there is a problem, which it begins to investigate, this response ramps up in response to the perceived threat. We only start to develop symptoms when the virus has become well established and self sustaining and many of the symptoms occur as a result of the bodies defences becoming increasingly aggressive. Many of these defences are also highly variable in the human population.

Many viruses are rather slopping in their reproduction, they rely on numbers rather than quality, a large number of the viruses will in fact be incapable of causing infection. The ones that are have to run an incredible gauntlet of defences and then have to reproduce fast enough to overcome the first defences and establish an infection before the adaptive immune system develops specific antibodies.

So with all of that the possibility of a single virus particle causing an infection is about as likely as them developing microprocessor technology, its well established that in virtually any infection a specific dose of the pathogen is needed, this dose varies with the pathogen, the method of infection and the host.

The links don't describe a single virus causing disease, they describe a single genetic variant of the pathogen becoming dominant, this is entirely predictable, the genetic variants in a population most able to establish infection will be the ones to succeed.

Sorry I just found this which is much more specific to Covid 19

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BillTre
My understanding is that if someone is exposed to a larger initial viral load then they are likely to have a more severe infection. That is why sick rooms should be well ventilated with fresh air to create dilution. Have a look at the many excellent videos by Dr John Campbell, such as:

OmCheeto, Laroxe and Vrbic
Laroxe said:
Yes, the type of virus is all important and then there is the issue of how the person is exposed to the virus, there are lots of barriers in place. ...
Thank you very much for the exhausting explanation!

Questions

1. How long does it take the Coronavirus to replicate? I am thinking a min or two.
2. Autoimmune system - Per article - the body's first response is general antibodies or general defense behavior. However, it seems that after some period of time an antibody is acquired that is somewhat specific to the virus. What is the process that is going on here? How is a specific antibody manifest in a relatively short period of time?

Phil Core said:
However, it seems that after some period of time an antibody is acquired that is somewhat specific to the virus. What is the process that is going on here? How is a specific antibody manifest in a relatively short period of time?