Governments have issued a lot of advice and have introduced many regulations in response to the COVID-19 pandemic caused by coronavirus SARS-CoV-2. Most of the time, the governments also tried to explain what they are trying to achieve. However, watching press conferences I have not heard much explanation of how the measures are supposed to work. Below, I present an analysis based on common sense, basic laws of physics, chemistry and biology, and easily looked-up information. Since the premises are firm, everything that I conclude should be correct, as long and I avoid all the logical fallacies. If you find any, please let me know. (You can find my address at the bottom of this page.)

1 Basic information

In December 2019, a new disease, now labelled COVID-19, was identified. It is caused by a coronavirus SARS-CoV-2. The genetic information of this virus is carried in RNA, which is enveloped in a membrane that is covered in glycoproteins. They are 60 to 140 nm in diameter. The RNA carries information that the virus needs to replicate itself inside a cell. The glycoproteins interact with receptors on cells and help the virus to enter.

Once a person is infected, the coronavirus multiplies in their body and the person becomes infectious. The infection exits the body in body fluids. The virus enters new hosts mainly through mouth, nose and eyes, possibly damaged skin or other body orifices.

2 Probability of being infected

Statistics and assessing probabilities is one of the places, where common sense tricks us more than it helps us. However, we should be able to identify what increases or decreases the probability.

During exposure there are three main factors that affect the probability of infection happening:

  • Susceptibility of the person
  • Point of entry of the virus
  • Amount of the virus

I am not going to discuss points one and two. Point one we cannot usually affect, unless there is a vaccine. And it is hard to imagine that you can choose the point of entry for the virus. If you had the choice, you would prevent it from entering from as many point as you can. However, with the amount of virus, it is safe to say that the more virus we get exposed to, the more likely we are to get infected.

I must emphasise that this is all playing a probabilistic game. You cannot eliminate the probability of being infected completely,1 just like you cannot be absolutely sure that you got infected (immediately after the event).

3 What destroys the virus?

Let us begin lightly with a comic that, however, has an important message.

xkcd comic: Cells

I am going to consider what kills the virus outside of the human body, which has completely different requirements to killing it inside (which should be done only by immune systems and approved medicinal drugs).

  • Time

    Virus is not a living organism, which means it will not die of old age, nor can we starve it to death. However, given enough time, it will eventually degrade. Ideal conditions would be damp (or even in liquid water) and enclosed (without fresh air).2 What destroys the virus could be UV light from the sun, free radicals from the air, chemical degradation from surroundings, enzymes from other organisms, possibly drying out. (Given that the virus is enclosed in a membrane full of proteins, one could imagine that drying this system may irreversibly damage it.) It may seem trivial, but smooth surfaces that look clean will also likely be unfavourable to the virus. However, even that probably means that they would be safe after at least few days, if stained by the virus. (And it also depends on how exactly it is stained. A virus could survive very long inside a water droplet which is inside grease that protects it from drying or air contact.)

  • Alcohol disinfections

    Alcohol (ideally ethanol) based disinfections denature the proteins of the virus and destroy its membrane.

  • Detergents

    Detergents (soaps belong to this category) may also dissolve the membrane and denature the protein. However, the main reason why we wash hands with soap is to remove the virus from our hands. Some of it may get destroyed as a bonus. I have not investigated whether strong detergents can actually be used as disinfections and definitely do not treat soap like a disinfection.

  • Bleach

    Bleaches are usually based on hypochlorite or other chlorine compounds. These are very aggresive chemicals that destroy pathogens by oxidising their molecules.

  • Ozone, UV light

    Professional dissinfections may be carried out by ozone or UV light, which are probably most harmful to the genetic information fo the virus, but could affect other molecules as well

And there will be many more options. For everyday use, we can use time, alcohol disinfections or bleach to destroy the virus and detergents to remove it from surfaces.

4 How is the virus transmitted?

There is a limited number of ways in which we can come into contact with body fluids of other people. The main ones include:

  • Direct contact
  • Surfaces
  • Droplets
    • Large droplets
    • Aerosols

4.1 Direct contact

This point is self evident and direct personal contact can transfer the largest amounts of virus. Usually, it is assumed that people in the same household will inveitably get infected if one person becomes positive and we may not be willing to sacrifice human contact with our partners and children, which is why governments do not advise against contacts within a household. Outside of a household direct contact can usually be easily avoided.

  • How not to infect others

    Make sure not to touch others, even by accident.

  • How not to get infected

    Do not touch other people whenever possible. If a contact already occurs, it is important to realise that the virus does not enter through healthy skin. Therefore, it is of utmost importance to wash one’s hands after accidentally touching another person that might be infected. If running water is not available, an alcohol-based (ideally ethanol) disinfection may be used. Make sure you do not touch anyone else, anything or any other part of your body. It is true that what you touch can be washed, but if you touch your cheek, good luck with washing it such that the infection cannot get into your eyes, nose or mouth.

4.2 Surfaces

Possibly the sneakiest culprit in the transmission are sufaces. An infectious person will deposit large amounts of the virus on everything they touch or cover with droplets. Another person touching the same surface and then accidentally transferring the virus to one of the entry points into the body (eyes, nose, mouth) can easily get infected.

  • How not to infect others

    Make sure not to touch surfaces (apart from your shoes on the ground of course). You can also deposit virus on surfaces via droplets as described below.

  • How not to get infected

    Do not touch surfaces. If you have to or happen to touch a surface do not touch anything else and wash or disinfect your hands. You may consider wearing gloves, but there are important points to be made.

    • Gloves decrease the amount of virus getting on your skin. This is useful if your skin is damaged and to make washing hands more effective. (You still need to wash hands after taking them off, though.)
    • You need to great care in how you remove gloves and dispose of them to make sure you do not spread the infection.
    • Gloves make you more conscious of what you touch and may prevent you from touching your face.
    • You can transfer virus from place to place on your gloves, possibly even better than on your skin.3 Therefore, still make sure you touch things and people only when necessary.

4.3 Droplets

An infected person will emit small droplets of saliva and mucus when coughing, sneezing, talking or even just breathing. We can distinguish these by size into two broad categories: large droplets and aerosols. This divison arises naturally from the way particles behave when moving through air. Of course, the size of droplets is continuous and smoothly varies between the two.

4.3.1 Large droplets

The larger droplets, when emitted from human body, exhibit what we could call ballistic motion, i.e. a motion in a relatively straight line. These droplets will be mainly ejected when a person coughs, sneezes or talks. Since these droplets are large, they can carry large amounts of virus particles. However, since they follow a ballistic curve, they will probably fall to the ground within a few meters. At the same time, they are directional, i.e. it is difficult to spit behind your back (in the absence of wind).

  • How not to infect others

    Firstly, you should stay as far from other people as possible. One can spray droplets only to a certain distance. If closer distances are unavoidable, then the best way to catch large droplets is to cover your nose and mouth. This can be done with (any) face mask or a face shield. If these are not available, even coughing and sneezing into a tissue or simply away from others could make a difference since these droplets are large and directional.

  • How not to get infected

    As always, stay as far from other people as possible. Apart from that, it is much more difficult to defend yourself against droplets than to protect others from your own. When someone sprays you with droplets, ideally you would have a face shield to protect your mouth, nose and eyes, and potentially gloves. Having a face mask and goggles should also shield you to some degree. However, also your clothes and nearby surfaces could become infectious, which makes things more difficult.

4.3.2 Aerosols

For small droplets, gravity and inertia are negligible compared to the interaction with air and they move in a chaotic diffusive manner. Every time a person breathes out, aerosol is created (in larger amounts, when person breathes heavily, speakes, coughs or sneezes). Since these droplets are small, they carry smaller amounts of virus, but could be infectious nevertheless. Because gravity is less important, these droplets will not drop to the ground easily, and can hang around in the air for a long time.

  • How not to infect others

    Stay away from people. If not possible, to remove aerosol from air you should force it through a filter. Normal face masks, also known as surgical masks, are not made to seal very well, which means that a great part of what we breath in and out will not get filtered. Therefore, ideally you would wear a respirator.


Respirators are face masks that are made to seal and that have a guaranteed filtering effect. The better respirators used during this pandemic are FFP3 or N99 which must stop 95% of particles larger than 300 nm. How comes that they protect from a virus, whose size is 60 to 140 nm? Firstly, no respirator offers 100% protection. Secondly, the virus moves through air in droplets, not as individual particles. Thirdly, even if the size of the pore is larger than the droplet, forcing the droplet through this mesh will deposit it on the fabric with some probability.

  • How not to get infected

    Stay away from others, especially indoors. The aerosol will diffuse all around the person. This may not be a huge problem outside, but inside the aerosol can concentrate. One more thing to realise is that the concentration of the aerosol will fall roughly with the square of distance (as it diffuses into a sphere around the person), which means that if you are one step away, making just one more step will probably make an order of magnitude difference in the propability of infection.

    It is very difficult to protect yourself from aerosol. Most intake will be from directly breathing it in. Ideally, you would wear a well fitting respirator. Other face masks may help, since some of the air will still go through some kind of filter, but air will also go around. To protect your eyes you would need sealing goggles. Overall, the best protection would be by a full face mask, but here we are getting to the point, where you might as well wear a full decontamination suit.

Diffusion and convection

While it is true that aerosols move through air via diffusion, which is quite slow, we must not forget that air itself moves very easily. Bulk movement of a fluid is called convection and, in most situations, it is convection that spreads aerosols across long distances. It is tempting to say that you observe diffusion, when watching a tea bag infusing in a cup. However, most of what you see is convection. The water cools down unevenly throughout the cup, leading to differences in densities and to currents equalising these differences. However, it is possible to observe mostly diffusion if we carefully drop a tea bag to the bottom of a cup filled with water. Then, if we are lucky, we would observe only a coloured band at the bottom of the cup. Such cup can be left for many hours and all that is observed is gradual smearing of the band upwards.

To get an idea of how quickly an aerosol can reach you in closed spaces, you can imagine a person lighting up a cigarette. It usually takes only a little while until you smell it. An aerosol of droplets will behave similarly.

(Thanks to Jáchym Sýkora, who suggested this addition)

5 What will common sense not tell us?

I am not an expert (a dangerous thing to say) and I am not trying to give advice. I tried to make sense of the various measures that are taken by governments. I reached some conclusions and we can learn some lessons.

  • Distancing helps
  • Washing and disinfecting hands helps
  • Not touching things, others and yourself, helps
  • Covering one’s face helps
  • Face masks protect others more than they protect you, so be as distant as you can even when wearing them.
  • Since we are playing statistics, if you cannot follow the advice fully, do so at least partially. It is all about decreasing the probability of transmission. E.g. If you cannot stay the recommended 2 m apart, do at least what you can. If you do not have a face mask, cover your face with something else. If you cannot cover your face, face away from people and couch and sneeze into tissues.

However, it is important to realise what one does not know, which is perhaps even more important than the knowledge itself.

I do not know how much the different modes of transmission contribute to the overall infection rate. I do not know how much each of the measures affects the rate of each mode of transmission. Maybe nobody knows; hopefully some people have at least a rough idea. It is up to scientists in the appropriate fields to make studies that try to answer these questions. And it is up to the politicians and officials of institutions like the WHO to look at the evidence and give advice. However, it is always comforting to see the rationale behind a received piece of advice that is given to you.


  1. Apart from deserted island scenarios. 

  2. The absolutely ideal conditions would of course be those used to store such specimens in a lab, i.e. rapidly frozen using liquid nitrogen to avoid formation of ice crystals and kept at −80℃. 

  3. Our skin is covered with RNAses, proteins degrading RNA. It is one of the antimicrobial mechanisms of our body. This means that to a very limited extent our skin disinfects itself from some microbes. Never count on this. (b.t.w. this is an annoying thing if you need to work with RNA in a lab. It is very easy to destroy your sample by touching something.)