This great article in Dutch is an interview with doctor, epidemiologist and professor emeritus Menno Jan Bouma. Below, I am going to copy and paste a Google-generated English translation which is very intelligibile.

However, before that, I am going to summarise what he says about how viruses such as coronaviruses actually spread:

  1. They spread through droplets; they leave a host in droplets and still wrapped up in these droplets they need to reach the new host.
  2. The smallest of these droplets are called aerosol. These are the most dangerous because, unlike heavier droplets which fall on the ground within seconds, the aerosols can remain suspended in air for longer, and thus they can be breathed in by people.
  3. The virus really becomes dangerous if it reaches the lungs, where the immune system is the weakest.
  4. If the virus reaches mouth and nose in a bigger droplet (non-aerosol), the overwhelming majority of times it will get stopped by the immune system defences present in nose and throat.
  5. If the virus reaches mouth and nose in aerosols, there are far greater chances to get straight through to the lungs.
  6. Coronaviruses are seasonal and thrive in cool, dry air. In these conditions, aerosol circulate for much longer.
  7. Because of the above, enclosed non-ventilated spaces are the most likely places where one can get infected.
  8. Ventilation and humidity are important, and most likely to get rid of any traces of the virus.

Then some recommendations:

  1. Keep your immune system in good shape; take extra doses of Vitamin C and D; take also Vitamin P (euphemism for pleasure, that is, take care of your psychological state, meet with friends, have fun, stay psychologically healthy)
  2. Avoid super-spread events, that is, avoid staying long hours with people indoor in a poorly ventilated space

Then final question: what about washing hands, sanitising and social distancing/no touching?

  • Those measures are perfect for tackling viruses such as diarrhoea viruses. These viruses travel from host to host through hands, through surfaces, and through touch. Indeed, if you want to deal with these viruses, it is very good to disinfect everything and to keep your distance and not touch each other.
  • But now we know that coronaviruses have a different strategy of traveling from host to host, and of invading hosts. They mainly spread by air. Compare it with cigarette smoke: it hardly spreads through hands or surfaces. Try comparing this to blowing cigarette smoke on your hands, and then give another one a hand or a hug. The chance that your cigarette smoke will end up deep in the lungs of the other person in this way is minimal. Coronaviruses need a lift of aerosols in order to get deep into the lungs where our body has less resistance. The measures washing hands, disinfecting, keeping your distance, no shaking hands and no cuddling are therefore not or hardly effective when you want to tackle viruses such as flu viruses or coronaviruses.

Full article in English

Doctor, epidemiologist and professor emeritus Menno Jan Bouma lived in Oost for years. As a guest lecturer he gave lectures at the Tropical Institute in East. He now lives in Ireland, but still likes to come to the East on a regular basis. He devoted his life to researching the ecology and distribution of viruses and other pathogens. In this time of the coronavirus, I would like to know a little more about viruses. So that’s what I’m talking to him about. 

Interview | Ruby Tilanus

You worked at the Tropical Institute in London, but came back to teach at our Tropical Institute in East. How was that?

Awesome. There are only a few places in the world with an enormous amount of knowledge about infectious diseases. The Royal Tropical Institute in East was one of them. Unfortunately, in the 1960s, many thought we had mastered infectious diseases, and in and after the 1980s, the Tropical Institute was largely cut back. The library has been closed. Specialists, collections and books disappeared. Much knowledge has thus been lost. And where knowledge is lacking, fear lurks.

That knowledge could now have come in handy in tackling the corona virus. 


Fortunately, you are still here and you want to share your knowledge about viruses with us. First of all, tell us what is a virus anyway?

A virus is not a “living” being like you and I are. A virus is not ‘alive’.

A virus consists of genetic material (DNA or RNA) surrounded by a packaging (a shell). A virus cannot do anything without a ‘host’ or ‘hostess’. It’s as dead as a piece of plastic. It doesn’t show any sign of life.

But if a virus is lucky enough to enter cells in a host’s body, it will behave like a hibernating bear.

The virus uses its host’s nutrients to reproduce itself. That is the only thing a virus can do and a piece of plastic cannot. That is the only thing that a virus itself, actively, can: reproduce itself. Although: itself, active… for this a virus does need a host.

What happens to a virus after it reproduces?

After reproducing, the new young viruses start looking for a new host.

That is where such a young virus has to travel to and penetrate. That is quite difficult for the virus, since it cannot do anything on its own, except reproduce itself. He has to travel passively or hitch a ride, and then try to penetrate somewhere. If that is successful, the virus feast of reproduction can take place again. So the virus cycle is: travel, invade, reproduce itself, travel, invade, reproduce itself and so on. A virus cannot or does not do anything else. Because a virus is not ‘alive’.

How do viruses do that: passive travel and passive entry? A virus has no wings, no legs and no webbed feet to travel, does it? And a virus cannot just enter through your skin, can it?

Right! A virus has no wings, no legs, and no webbed feet to travel from one host to another. And indeed, a virus cannot just enter through your skin. Your skin is literally an impenetrable barrier to a virus.

Viruses have evolved various strategies to travel and invade their hosts. The way a virus travels is directly related to the way the virus enters the body of its host after its journey. So you always have a way of traveling and a way of penetration that are linked to each other. Each virus type specializes in at least one of those strategies. On that basis, you can divide all viruses out there into roughly three different main groups.

Please tell me a little more about those different main groups of viruses…!

You can give the main groups the following names:

– ‘Opportunists’: they enter the body through damage such as a wound or a blood transfusion (think of the AIDS virus);

– ‘Survivors’: they follow the path of food (think of diarrhea viruses);

– ‘Kites’: they hitch a ride with the air that is inhaled (think of flu viruses and corona).

First you mention: Opportunists (such as the AIDS virus). What is the strategy of Opportunists?

An opportunist waits until he finds an opening somewhere in the skin of the new host, a wound for example. Then he waits until his old host makes direct contact with that wound with his blood or with his semen. And then it enters the body of the new host through that wound. Opportunists can also enter via an insect sting (sting).

And Survivors (such as diarrhea viruses)? What is their strategy?

Survivors can withstand the extreme acidic gastric juices and the extreme conditions of the intestines, and can survive there. They reproduce in the intestines. If an old host does not wash his hands after defecating, a tiny amount of feces can get on the hands of the old host, which can then end up on a table, elevator button, or other surface, for example. If the new host touches that surface, or shakes hands with the old host, the Survivor may end up on the hands of the new host. If the new host then licks his fingers and does not wash his hands before eating, the Survivor can pass through the mouth into the stomach and intestines of the new host.

What interests me especially in this time of corona: what is the strategy of Vliegers (such as influenza viruses and corona)?

The strategy of kites is to hitch a ride with small droplets that the old host lets out through the mouth. When the old host exhales, talks, sings or coughs, small droplets come out of his mouth. Kites try to hitch a ride with those little droplets. A lucky kite will leave the body of the old host in a tiny droplet. It then travels through the air to a new host and enters the body along with the inhaled air. There he tries – still packed in a drop – to hitchhike up to the alveoli, deep in the lungs. Deep in the lungs, the lung wall is very thin: only one cell layer thick. Our immune system is weak there: far fewer virus particles are needed to make you sick.

How small are those droplets really? 

The droplets are all small. But some droplets are many times smaller than other droplets. We call the smallest droplets ‘aerosols’ and they can fly.

The larger droplets do not float in the air for long. They fall to the ground within seconds due to gravity. They cover a distance of about half a meter with normal breathing to a few meters with a strong cough. To reach the new host, the Flyers must get to the nose or mouth level of the new host. However, the larger droplets soon bend towards Earth due to gravity. The aerosols remain floating in the air for a very long time. If not properly ventilated, they can continue to circulate in the air for hours. They can cover enormous distances.

Okay, this is the story on the part of the viruses. But what about our side of the story? What is our defense against viruses?

Man has been living with viruses since its inception. We and our distant ancestors in the animal kingdom evolved along with the viruses, as it were. There are many forms of innate and acquired defenses that have developed over time. Antibodies are just one of them. We have developed appropriate defense strategies for each of the different strategies of viruses for survival (Opportunists, Survivors and Airmen).

Tell us: what is our defense strategy against Opportunists (such as the AIDS virus)?

Our oldest defenses may have been developed against Opportunists. We have skin. Our skin blocks almost everything harmful that our body wants to enter. When we have a wound, our body is very well able to heal that wound very quickly.

And what is our defense strategy against Survivors (such as diarrhea viruses)?

We have developed a very sour stomach for them. Our gastric juice damages almost all viruses, so that they can no longer reproduce. Most Survivors also do not survive our gastric juice – only the few who do get the chance to reproduce.

And now the most important thing: what about our defenses against Kites (such as flu viruses and corona)?

To get deep into the lungs, a Kite must first pass through the mouth or nose. We all have multiple defense mechanisms in our nose, mouth and throat, including immune cells. An entire army with a vanguard, a rearguard and many layers of commanders is ready to stop the Flyers.

Most intruders get stuck in the labyrinth of mucous membranes and small hairs in your nose and are defused here. The mouth and pharynx also participate in the defense against the Airmen. If they get into the stomach, they don’t stand a chance there.

Does it matter whether a Kite enters through a large or a small drop (aerosol)?

Yes. If a Kite hitches a ride with a small drop (aerosol), the chance of breaking right through the defense line is much greater than with a large drop. Just as small fish are much more likely to slip through the mesh of a net than large fish. Together with the inhaled air, the Vlieger can also – if he is lucky – fly deep into the lungs. After all, the drop is so small that – with a bit of luck – it can go straight into the lungs. The Vlieger virus then hitches a ride with the aerosol.

When a Kite hitches a ride in a big drop, he has a much harder time at the entry of the new host. The chance that he will be stopped at the port (in the nose, mouth or throat) is then maximum.

What exactly happens in the nose? 

Everything in the nose is designed to prevent viruses, bacteria and other germs from entering our body. In addition to the nose, the mouth and throat are also extremely adept at this. Our defenses are ready to deal with intruders. In most cases we manage to effectively stop intruders (such as viruses and bacteria) in the nose and make them harmless. In the meantime, we have ‘become acquainted’ with the intruder. This means that the next time the same intruder shows up at our front door again, we can intervene all the faster and more effectively. You can compare that to a kind of standing up and getting acquainted at the front door: our body becomes acquainted with the previously unknown virus, and can start to build up resistance and make antibodies against the virus.

You have researched the role of the seasons and the weather on the spread of infectious diseases. Does that also play a role in the spread of corona?

Yes, most infectious diseases are seasonal diseases. For Vliegers, this mainly has to do with the temperature and the humidity. Kites such as flu viruses and corona thrive in cool, dry air, while Survivors dry out faster and die in dry air. It is not yet entirely clear whether the humidity is bad for the virus itself or for the life of the aerosol. In moist air, aerosols may quickly deflect to the earth. In dry air, aerosols can keep circulating for much longer. In the summer we have seen many cases of corona in the meat processing industry: there the air is kept cool and dry and in order to save costs, the outside air is insufficiently refreshed: a ‘mecca’ for Vliegers (aerosols). In winter we heat our houses and we are more indoors.The air dries out and we live together in a smaller space. You can then ingest a larger dose of viruses into the lungs, your most vulnerable place. That makes flu and corona real winter viruses. Savings on heating (from draft excluders to intelligent systems that only supply fresh air when it is really needed) is usually at the expense of fresh and more humid outdoor air. So we have to be careful with that!

With the winter season approaching, I advise everyone to ensure good humidity in the house. By putting a bowl of water on several radiators, for example.

Do you think it is wise to meet up with people outside? 

Meeting outside is indeed sensible. You can compare the behavior of aerosols with the behavior of cigarette smoke. If you smoke a cigarette outside, your smoke will dissipate quickly and will not bother others except when you blow them in the face. It is the same with aerosols. If you carry the corona virus and you meet up with people outside, they will not be bothered by your aerosols. Except when you blow them in the face.

And what about ventilation exactly? Does that make sense?

Yes, that certainly makes sense. Compare it again with cigarette smoke. If you go to smoke a cigarette in my living room and I don’t ventilate properly, well, I can still smell your cigarette later on. But if I open the windows opposite each other for a while, the smell will disappear in no time.

It is the same with aerosols. If you ventilate well and open the windows together, they are gone in no time. Especially in winter, the peak season for the virus, fresh outside air is very important.

What more can we do to contain Kites such as the corona virus?

Very important in the fight against all infectious diseases is: ensure that your natural defense system can function properly. Think of a healthy diet, sufficient exercise and enough sleep. Take an extra dose of vitamins C and D daily for strengthening: these vitamins help your immune system.

Also ensure psychological well-being. We call this ‘vitamin P’ where the P stands for pleasure. Do things that make you happy. Dance, make music, meet friends, go out.

And always avoid super spread events.

What do you mean super spread events? Tell me more about it…

The coronavirus can spread super fast during so-called ‘super spread events’. These are events where many people spend hours together in a poorly ventilated space. Think, for example, of Après-ski bars or carnival cafés. Only one person needs to carry the corona virus. That one person unnoticeably blows many aerosols into the air. Probably some coronavirus carriers do this more intensely than others. All those present then inhale these aerosols for hours. The virus has a very good chance of invading many, in numbers that our defenses cannot handle.

I don’t hear you about washing hands, disinfecting, keeping your distance, not shaking hands, and not hugging…!

Those measures are perfect for tackling Survivors such as diarrhea viruses. Survivors travel from host to host through hands, through surfaces, and through touch. Indeed, if you want to deal with these viruses, it is very good to disinfect everything and to keep your distance and not touch each other.

But now we know that corona is a Kite. Kites have a different strategy of traveling from host to host, and of invading hosts.

As said: Kites mainly spread by air. Compare it again with cigarette smoke: it hardly spreads through hands or surfaces. Try blowing cigarette smoke on your hands, and then give another one a hand or a hug. The chance that your cigarette smoke will end up deep in the lungs of the other person in this way is minimal. Kites need a lift of aerosols in order to get deep into the lungs where our body has less resistance. The measures washing hands, disinfecting, keeping your distance, no shaking hands and no cuddling are therefore not or hardly effective when you want to tackle Kites such as flu viruses or corona.

But Menno, isn’t that diametrically opposed to what Mark Rutte asks of us ?!

Partly yes. But Rutte has also asked us several times to use our common sense …

I’ve heard you say, “Viruses are like burglars. You have to stop them at the front door. ‘ What exactly do you mean by that?

There is a big difference between a burglar who is stopped at the front door, and a burglar who enters your house for a moment and then walks out, and a burglar who is going to settle in your house and live there. It is the same with viruses. If you ‘stop a virus at the front door’, that is, if you stop a virus in your nose, mouth and throat and make it harmless, then you are doing well. You will literally get to know the virus up close, so that you can react even faster next time. If you briefly inhale a virus and then exhale it again, there is not much going on either. But if a virus settles deep in your lungs and starts to reproduce there, yes, it can make you very sick.

We are currently seeing a huge increase in the number of corona infections. Also here in East. How do you feel about that?

The infections are now measured with the so-called ‘PCR test’. This test examines whether you have the coronavirus in your nose, mouth or throat, or leftovers of the dead virus if the infection has already passed. Many people who have the virus in their nose, mouth or pharynx are busy ‘getting acquainted and standing by the front door’. As I just said: in normally healthy people with a normal immune system, there is a whole army of immune cells ready in your nose, mouth and throat. That army is introduced to the virus, makes the virus harmless, and at the same time has gotten to know the virus and so it can react even better next time. I personally think that the word ‘contamination’ is therefore not so well chosen. I would rather speak of ‘acquaintance’. Yes,it is true that the number of ‘acquaintances’ with the virus is increasing (enormously).

But isn’t it really bad that the virus is in the nose of more and more people?

Yes and no. People who have come to know the virus and have managed to survive at the front door function as a kind of ‘living shield’ for all the vulnerable in our society. I am talking here about the famous ‘group immunity’. As a community you need some time to build up group immunity. Young, vital people generally have little or no problems with corona. It almost always remains with them sniffling a little. In the Netherlands there is a great sense of togetherness, and the care for the elderly is wonderful. A nice matching slogan could be: “Take care of your grandmother: make sure you have had corona.” The more young, vital people have become acquainted with the corona virus and have kept the virus standing at the front door, in other words: have become immune to corona,the more favorable that is for our fellow human beings with very weak defenses.

Which brings me to the question: how deadly is corona really? Do virologists and epidemiologists have a standard yardstick for that?

In the Netherlands we live with 17 million people. People are born every day, and people die every day. That is a natural process. If more people die than average, we speak of ‘over-death’. If fewer people die than average, we speak of ‘under-mortality’. In the Netherlands we had excess mortality in the months of March and April 2020. After that there was a period of under-mortality in our country for months.

We express the lethality of a virus with the IFR, the Infection Fatality Rate. That is the percentage of people who have been in contact with the virus who die from the virus. Corona has an estimated IFR of 0.5 percent or less. This can be compared to the IFR of a medium to severe flu.

Yes, corona is a new virus. But I want to make a case for moving from fear to trust. We count the brothers and sisters of the corona virus under our ‘medium winter flu’. Global panic is not necessary. People generally deal more rationally with the much more severe infections that you see in the tropics.

But in the US, a lot of people die from corona, right?

That’s right. In areas with a high mortality, it is striking that the socially deprived part of the population in particular is dying. This part of the population has an increased risk of a serious course of the disease, such as obesity and diabetes. Bad food, bad air, bad housing, bad (clean) water supplies all contribute directly or indirectly to a weakened immune system.

At the moment there is a lot of fuss about mouth masks in schools. What is your opinion about that?

Mouth masks may be of some help in preventing the virus from passing on to others if you have COVID-19 disease. In schools, however, the advantages seem smaller than the disadvantages: risk of breathlessness, concentration problems, headaches and possibly brain damage.

How do you think corona will continue in the coming months and years?

It is expected that we will have an increased risk of aerosols in the months of November to April. If we protect the vulnerable in society well, and if we ventilate well, the second wave will not be as intense as the first wave. After all, many people have already become acquainted with the virus. Also in the future, corona will keep coming back in the winter months. But it never has to be as fierce as last spring, when the virus was completely new to everyone.

You have devoted your life to studying infectious disease management. What do you notice worldwide in the approach to the corona virus?

Cutting back on care and slimming down the hospital bed stock has contributed significantly to the fear and panic measures to avoid being overrun by the volume of patients. Unfortunately, this does not only apply to Amsterdam East and the Netherlands.

Because the benefits of the cuts are often not evenly distributed, a large part of the world’s population has unfortunately become even more vulnerable to following a healthy lifestyle.

As far as the approach is concerned, I think that a study of the older literature and targeted research based on this can contribute to an effective policy. A policy devoid of fear, panic and the need to sacrifice personal freedoms.