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I'm not a biology student at all, but I'm trying to get a clearer picture on what is meant by "virus cannot survive after a certain period".

To my understanding, a virus cannot be killed it can only be inactivated (for example, by means of heat and lowered pH).

So I really don't understand how can virus get inactivated by just being on a certain surface for an extended period.

I've come across numerous articles which told me virus won't survive on a surface. They usually focus on comparing the survival time between different types of surfaces but never touch on how the virus gets inactivated.

Can someone help me understand the "how"?

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    $\begingroup$ Related: 2019 Novel Coronavirus (nCoV) lifetime outside an organism $\endgroup$
    – dotancohen
    Commented Jan 29, 2020 at 12:55
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    $\begingroup$ "To my understanding, virus cannot be killed" This is just because they're generally deemed not to be alive in the first place. $\endgroup$
    – ceejayoz
    Commented Jan 30, 2020 at 13:44
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    $\begingroup$ It does depend on the surface, and the virus, different ones are destroyed in different ways. A virus survives in the same way an antique car survives, as in it still exists in good enough shape to function. $\endgroup$
    – John
    Commented Jan 30, 2020 at 20:33
  • $\begingroup$ I am thinking virus can be degraded by lots of extracellular enzymes (e.g. proteases) which microorganisms are constantly secreting. That's how elements locked up in "biomass" cycle again. And that explain why virus survive more poorly in moist and hot environment where microbial activity is the highest. I could be wrong but just saying. $\endgroup$
    – y chung
    Commented Mar 13, 2020 at 19:56

5 Answers 5

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Many important viruses are coated with a lipid envelope and rely on this to enter the host cell. This envelope is fragile - it's similar to a soap bubble - and it can be disrupted in many ways. Lipids will oxidize in air over time and this will degrade their ability to maintain an envelope. Surfactants such as soap or solvents such as alcohol will disrupt an envelope quickly. Even if the genome inside survives, if the envelope is disrupted the virus won't be able to infect cells. The exact mechanism of inactivation likely varies a lot from virus to virus, and hasn't been studied extensively. This paper found that Hepatitis C virus RNA survived alcohol treatment, but lost infectivity, presumably due to envelope disruption. In contrast, heat treatment (80°C) destroyed both the envelope and the RNA.
Another paper found enveloped viruses survived being dried out a shorter time than non-enveloped viruses. (5 days vs. weeks)

Speaking generally, the environment outside a body is hostile! If you were taken out of your home and dropped somewhere in the wilderness, there's many different ways you could die, and it's more remarkable if you survive. A similar situation arises for viruses.

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    $\begingroup$ +1 To be fair I don't think the last paragraph is a good comparison, as a virus is just a bunch of molecules and not a living thing. And I think it's from this understanding that the original OP asked his question. If you put a table outside on a surface it won't get destroyed either. Obviously the answer is "viruses are relatively fragile, because evolutionary the advantages of surviving away from organisms that could replicate them was fairly pointless" . Does make me wonder whether with the increased population density of human organisms there will be a point where it's worth surviving. $\endgroup$
    – David Mulder
    Commented Jan 29, 2020 at 12:08
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    $\begingroup$ The point I was trying to make is that viruses are complex systems that have many potential points of failure. In that regard, they are closer to a living thing than a table, which can take a lot of damage and still function as a table. Maybe a better comparison would have been to some electronic device like a PC that you'd expect to stop working if its left outside. But, I expect rain would destroy it before anything else 99% of the time so it doesn't work the same way. $\endgroup$
    – timeskull
    Commented Jan 29, 2020 at 15:28
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    $\begingroup$ @DavidMulder: To give a better analogy, a virus is like a car that functions well in the city but will break down gradually if left into the wilderness, rapidly if it is thrown into a pond or the sea. Similarly, viruses 'survive' well in their hosts, but will gradually break down if left outside, and rapidly so in certain environments. $\endgroup$
    – user21820
    Commented Jan 30, 2020 at 4:03
  • $\begingroup$ @DavidMulder Even with the table, leaving it outside under the sun / rain / snow etc. versus leaving it in your home over a long period of time means different rates of degradation. $\endgroup$
    – John Hamilton
    Commented Jan 30, 2020 at 10:59
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    $\begingroup$ @DavidMulder I would argue that there's a fine line between a "bunch of molecules" and a "living thing". The death of a "living thing" is just what happens when a sufficient number of internal systems of the thing (which are themselves "bunches of molecules") are placed in hostile scenarios for a sufficient time for them to cease function. I would think the comparison of a virus on a surface to a human in a desert is apt (if incredibly simplified) - the concept of "death" might be different from a micro to a macro scale, but the ultimate result is the same for the subject. $\endgroup$
    – Abion47
    Commented Jan 31, 2020 at 21:01
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Ultraviolet (UV) light emitted from the sun has enough energy to break chemical bonds in DNA and RNA.

Some frequencies of UV light can cause damage in the DNA in skin cells that can lead to replication and expression errors, which lead to cancer (melanoma).

Similarly, UV can break up and inactivate the genomic payload of a virus:

Sunlight or, more specifically, solar UV radiation (UV) acts as the principal natural virucide in the environment. UV radiation kills viruses by chemically modifying their genetic material, DNA and RNA. The most effective wavelength for inactivation, 260 nm (55), falls in the UVC range, so-named to differentiate it from near-UV found in ground-level sunlight, i.e., the UVB and UVA portions of the spectrum, 290 to 320 nm and 320 to 380 nm, respectively (51). Nucleic acids are damaged also by UVB and UVA but with lower efficiency than by UVC radiation (64).

Surfaces exposed to sunlight would see partial to complete inactivation of virus particles over time.

There is a paper here that discusses using a safer frequency of UVC radiation as a biocide:

The biophysical reason is that, due to its strong absorbance in biological materials, far-UVC light does not have sufficient range to penetrate through even the outer layer (stratum corneum) on the surface of human skin, nor the outer tear layer on the outer surface of the eye, neither of which contain living cells; however, because bacteria and viruses are typically of micron or smaller dimensions, far-UVC light can still efficiently traverse and inactivate them13,14,15.

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  • $\begingroup$ So your saying natural sun light is enough to kill viruses. though it can take days/ hours dpending on the surface? Also that it only kills the viruses that are on the surface, and not behind something/ embedded deep inside? So a moist though thin hankie, would take longer to disinfect than a similar dry one? $\endgroup$
    – tgkprog
    Commented Jun 30, 2020 at 23:46
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    $\begingroup$ You'd need to measure how different UV frequencies get through layers of fabric, but people wear clothes and hats to protect against sunburn. This should perhaps give an intuitive sense to what extent a hankie can be sanitized. $\endgroup$
    – Alex Reynolds
    Commented Jul 1, 2020 at 2:54
  • $\begingroup$ Well I have used a hat and a hankie. A hat worked well. But a hankie made me feel hot very soon. I'm bald. I don't trust my intuition here. Was curious to know your knowledge or best guess? :-) $\endgroup$
    – tgkprog
    Commented Jul 1, 2020 at 14:46
  • $\begingroup$ I don't know the answer, but I suspect keeping a handkerchief in your pocket will allow it to accumulate a lot of bacteria and viruses. You could put your handkerchief on a clothesline to expose it to the sun, but then there are pollen and mold spores in the air, which are allergens for some people. I hope you find an answer. $\endgroup$
    – Alex Reynolds
    Commented Jul 1, 2020 at 19:07
  • $\begingroup$ okay thank you. hopefully multiple researchers will do a bunch of studies and answer these question in the coming months $\endgroup$
    – tgkprog
    Commented Jul 2, 2020 at 12:45
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Like other forms of life, viruses are composed of fragile molecules susceptible to damage from the environment. They may oxidize in the air, suffer mechanically stresses, be hit by UV light, etc. Metabolism in cellular life combats this effect by actively making repairs and by forming and maintaining protective barriers.

Viruses don't do that. Outside of a cell, they don't do anything. They don't have a metabolism. They can't make repairs. The damage accumulates until they can't work anymore.

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  • $\begingroup$ Welcome to Biology.SE! Your point about viruses not be able to make repairs is a good one, but answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. ——— Please take the tour and then consult the help pages for additional advice on How to Answer effectively on this site and then edit your answer accordingly. Thanks! 😊 $\endgroup$
    – tyersome
    Commented Feb 4, 2020 at 3:05
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For the record, I would like to place here a response that was received for How do viruses die outside of host cells?, which was deemed to be a duplicate of this post and closed.

This isn't very professional, but there are the basics. Viruses are obligate parasites, which means they can't live outside a host (in this case, a cell) for long. Viruses get damaged as everything else, the damage taking many forms, such as hydrolysis, photolysis, free radical damage, reaction with other molecules (like detergent), conformation changes, dehydration, maillard reactions, and changes induced by heat of labile molecules (like decomposition). All types of damaging processes happen at an increased reaction rate with increasing light and heat. Viruses have rudimentary repair mechanisms if any, so it's a matter of time before they become inactivated. A damaged virus entering the body wouldn't be able to do much harm if any, as their genetic information would be damaged beyond repair, and so, not able to be replicated to make more viruses. Their proteins and genetic bits would still trigger an immune reaction and immunize against intact and not very damaged viruses (which is why damaged viruses are used in vaccines). Those damaged viruses that remained outside just become microbe food.

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Virus as complex ordered system, without actively maintaning the order, the order will lose after time and finally lose the ability to infect the host.

Even the virus in a host body that is not destroyed by the immune system will "die" (means not functioning any more). It doesn't matter a single virus live or die, the important thing is the pupulation - if virus get replicated is more than that is dead in a period, the population will increase in the host body.

Infection is also about statistics, you need enough virus to get into the host body for them to successfully start the pupulation increasing process before they all become dead (killed by immune system or degraded in other ways)

We are not sure whether the virus particales outside host are all dead or not, only sure about the chance they infect the host after a time is low.

So another reason (besides detailed mechanisms) is that they are not replicating to sustain the population that is dense enough to easily infect the host.

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