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A simple mental model of a viral infection is that an infected cell emits a lot of virions and eventually dies. The emitted virions have a chance of infecting other cells. Nearby cells are at a higher risk of infection.

Based on this model, if one cell in my nose gets infected, I would expect a large part of my nose to be destroyed, as the infection spreads and destroys more and more cells in the same area.

simple infection model

This does not happen! I survived a number of infections and still have my nose. Why?

I know there are "flesh eating" bacteria. Why isn't this the norm for infections? Does a common cold virus or SARS-CoV-2 not infect a lot of cells within the same area?

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    $\begingroup$ Sorry for the childish question. I'd truly like to learn the answer. biology.stackexchange.com/questions/93782/… says a virus may infect around 0.3% of the cells in the human body. That could still be a visible piece out of my nose! $\endgroup$ Nov 12 '21 at 10:36
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    $\begingroup$ If you’ve ever had or seen a cold sore, that’s a viral wound. There are others, but not a great deal, and the reason is explained below. $\endgroup$ Nov 12 '21 at 16:51
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    $\begingroup$ Cells which are infected release interferons which, among other things, inhibit protein synthesis in nearby cells. This reduces the susceptibility of nearby cells and is likely one of the reasons behind tissue infection not perfectly matching your model. $\endgroup$
    – forest
    Nov 12 '21 at 23:32
  • $\begingroup$ How thick do you think your nose is, in cells? How many cells deep would a virus have to "dig", starting from the inside of your nose, until it popped out on the face side of your nose? $\endgroup$
    – Caius Jard
    Nov 15 '21 at 10:47
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    $\begingroup$ Faulty premise. Viruses certainly can and do cause wounds. Measles, Ebola, Herpes, Smallpox... the list goes on. All of them causing horrible lesions, pustules, and nasty suppurating wounds. $\endgroup$
    – J...
    Nov 15 '21 at 17:23
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A virus does not destroy that many cells before it is exterminated by the immune system or before the host dies.

Perhaps even more crucially, viruses typically target a very specific type of cell — those on the inner mucal surface of the nose in the case of cold or flu, those of the gastrointestinal tract in the case of stomach viruses, CD4 immune cells in the case of HIV, etc.

Update As an example of how much time it takes for a virus to eat a noticeable wound, one could take the extermination of the immune cells by HIV - although it does not look as a physical wound, it is one, in the sense that enough of the specific tissue is destroyed to cause a life-threatening condition. It takes about a decade(!) - from the initial infection to the immune system failure.

On the other hand, the lethal effect of typical respiratory viruses is typically via obstructions of the respiratory ways due to inflammation or secretions resulting from the immune response, or via creating suitable conditions for a more serious bacterial infection.

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  • $\begingroup$ Thank you! This makes sense. So the key is that the targeted cells are sort of rare? Even if the virus kills all these cells in my nose, the loss is invisible? I have no idea what my nose is made up of. But there are parts that I think are largely made up of one type of cell. (Muscles, brain.) Do viruses cause macroscopic damage in those tissues? $\endgroup$ Nov 12 '21 at 17:02
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    $\begingroup$ @DanielDarabos I think the point is that there are cascading secondary effects that cause more damage once a small number of cells are destroyed directly by the virus. Analogy: if you're stabbed, the wound is just a small hole, but you can lose much blood through it. $\endgroup$
    – Barmar
    Nov 13 '21 at 18:24
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    $\begingroup$ Another example of a possible local 'wound' is the anosmia (loss of sense of smell) that occurred frequently with early variants of covid. There's evidence suggestive that entry, infection and death of sustentacular cells (which support the olfactory receptor neurons) could be a major contributor to anosmia and thus a very specialized wound that, while not visible, has a directly notable side effect. ncbi.nlm.nih.gov/pmc/articles/PMC7488171 $\endgroup$
    – Dan Bryant
    Nov 15 '21 at 14:43
  • $\begingroup$ @DanBryant A very relevant example, thank you! $\endgroup$ Nov 15 '21 at 14:52
  • $\begingroup$ "it does not look as a physical wound" *look like $\endgroup$ Nov 15 '21 at 23:10
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The innate immune system is remarkably good at providing a first line of defense. Viruses don't just march into your body uncontested. They basically have to fight for every square centimeter of ground they claim, and even then, it's a race against time before the adaptive immune system pulls facial recognition on them and sends out hit squads which targets them directly.

That means that viruses aren't necessarily able to infect all of the cells in a particular area of tissue. While viruses have some defenses against immune systems, their very simplicity limits the tricks they can use. Flesh-eating bacteria, being much larger and more complex, have more tools at their disposal when it comes to attacking host tissue.

Kurzgesagt has some really good videos on the immune system:

The complement system, in particular, is especially interesting, because it's basically a free-floating chemical warfare system which passively protects virtually every part of your body 24/7. No signalling molecules or specialized immune cells are necessary for it to start interfering with foreign invaders.

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    $\begingroup$ Square centimeter seem huge $\endgroup$ Nov 13 '21 at 3:07
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    $\begingroup$ @theonlygusti: Change to square micrometre then. $\endgroup$
    – user21820
    Nov 14 '21 at 15:02
  • $\begingroup$ "Flesh-eating bacteria, being much larger and more complex, have more tools at their disposal when it comes to attacking host tissue." One of which being producing toxins (I suppose a virus theoretically could have genes that cause your cells to produce toxins, but they can't produce anything directly). $\endgroup$ Nov 15 '21 at 23:12
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Smallpox, now eradicated, caused severe lesions. In those who survived: the lesions often turned into scars.

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  • $\begingroup$ Welcome to Biology.SE. Please take the tour and then consult the help center pages for additional advice on How to Answer effectively on this site and then edit or delete your answer accordingly. In particular, note that answers should directly address the original question and are much more likely to receive a favorable response if they include supporting references (primary literature is best). A critique should be posted as a comment below the post, not as an answer. $\endgroup$
    – tyersome
    Nov 14 '21 at 17:27
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The other answers cover our immune system and the likelihood of viruses surviving it.

Regarding your missing nose:

Do you know how big skin cells are?
Do you know how many need to vanish to leave a visible defect?

A typical skin cell is about $30$μm² - that is really really really small.

Some math:

Using these numbers we can estimate how many cells need to die for a 1mm² defect:

  • $1$ mm² holds $1.000.000$ μm²

That fits about $33333$ skin cells.

To remove your nose though we need volume - so:

  • $1$ mm³ holds $1.000.000.000$ μm³

Assuming$^\bigstar$ square-ish skin cells, each cell is about $\sqrt[2]{30}^3 = 165$ μm³ "big".

A $1$ mm³ "hole" has $1.000.000.000$ μm³, that are about $6.06 * 10^6$ square-ish skin cells for a defect that I regularly make while shaving.

And all those damages are constantly under repair by your body.

Summarize:
The danger of you losing your nose from a viral infection is negligible. They tend to spread out and not stay localized, are constantly battled by your immune system and efforts of your body to repair/remove damaged cells.


$^\bigstar$ not squarish - but for simplicity's sake we'll go with that

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    $\begingroup$ I'm not sure this really addresses the thrust of the question. I interpret it as; "a virus that initially infects the nose can do enough damage to kill you. How can that much damage occur without the initial site of the infection being totally destroyed." (The answer really being that viruses don't fully conquer one area before moving on but cause small amounts of damage all over the place) $\endgroup$ Nov 13 '21 at 11:08
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    $\begingroup$ Those huge numbers have no meaning for exponentially multiplying things like viruses... $\endgroup$
    – wimi
    Nov 13 '21 at 14:23
  • $\begingroup$ (1mm)²/(30μm)² = 1,111 not 33,333 are you assuming that the skin cells are 30μm by 1μm? (I mean, they aren't going to be square either, but they presumably aren't that narrow) $\endgroup$ Nov 13 '21 at 23:14
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    $\begingroup$ ("losing" (not loosing - near "The danger".) $\endgroup$ Nov 14 '21 at 7:51
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    $\begingroup$ @PatrickArtner I can't edit to make that correction (30um² area rather than the currently 30um) to your answer as it's only one character $\endgroup$ Nov 14 '21 at 21:34

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