Home Overzicht Viruses are like burglars: You must holt them at the front door

Viruses are like burglars: You must holt them at the front door


Doctor, epidemiologist and emeritus professor Menno Jan Bouma lived in Amsterdam and the UK.  He lectured at the London School of Hygiene and Tropical Medicine, and as guest lecturer, at Amsterdam’s Tropical Institute in East. He now lives in Ireland. He has devoted his life to researching the ecology and distribution of human pathogens. In this time of the coronavirus, I would like to know a little more about viruses. So that’s why I am talking to him.

Interview | Robijn Tilanus

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

Awesome to be back! There are only a few places in the world with an enormous amount of knowledge about infectious diseases. The Royal Tropical Institute in Amsterdam was one of them. In the 1960s, many thought we had mastered infectious diseases, and in the 1980s and 1990s the Tropical Institute was largely stripped of its subsidies and glory. The library was closed. Specialists, collections and books disappeared. Much knowledge has thus been lost. And where knowledge goes missing, fear is around the corner.

That knowledge would have been helpful for the approach of the corona virus.


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

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

A virus consists of genetic material (DNA or RNA) surrounded by a cover (a coat). Without a “host” or “hostess”, a virus cannot do anything. 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 behaves like a bear coming out of hibernation. The virus uses its host’s nutrients to reproduce itself. That’s the only thing a virus can do and a piece of plastic can’t.

What happens to a virus after it has been reproduced?

After reproduction, the new young viruses start looking for a new host. That’s where such a young virus has to travel, and get in. That is quite difficult for a virus, because it cannot do anything on its own, except reproduce itself. It has to travel passively, in other words try to hitch a ride with something and then try to enter somewhere. If that succeeds, 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 do anything else. Because a virus is not “alive”.

How do viruses do that: passively travel and passively enter? A virus has no wings, no legs, and no flippers for travel, right? And a virus can’t just enter through your skin, right?

Right! A virus has no wings, no legs, and no flippers to travel from one host to another. And indeed, a virus cannot just penetrate your skin. Your skin is 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 entering that are linked to each other. Each virus type is specialized in at least one of these strategies. Based on that, you can divide all viruses out there into roughly three main groups.

Please tell me about these main groups of viruses…!

You can give the three 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 way of food (think of diarrhoea viruses);

– “Flyers“: 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?

Opportunists have a strategy: grab your chance! An opportunist waits until it 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. Once it has entered the new host, it can reproduce.

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

Survivors’ strategy is to resist the extreme acidic gastric juices and survive the passage through the stomach. They can now reproduce in the intestines. When an old host does not clean his or her hands after passing stools, a little bit can remain on the old host’s hands, and can end up on a table, a lift button or another surface. When another person touches that surface or shakes the old host’s hands, than the Survivor ends up on the hands of the new host. When the new host licks his fingers, or doesn’t wash his hands before eating, the Survivor can reach the gut again via the mouth and stomach of the new host.

Of course, what interests me most in this corona time: what is the strategy of Flyers  (such as flu virus and corona?

The strategy of Flyers is to hitch a ride with small droplets that the old host lets out through the mouth. That happens when the old host exhales, talks, sings, or coughs. Flyers try to hitch a ride with those little droplets, and a lucky Flyer leaves the body of the old host in this way. Through the air it travels to a new host, and enters along with the inhaled air. There it tries to reach the alveoli, deep in the lungs, where the lung wall is very thin: only one cell layer thick. Our immune system is weak there, so our resistance is less, and a few virus particles there can 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 metre with normal breathing to many metres 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 however, remain floating in the air for a very long time. If a room is not properly ventilated, they can continue to circulate in the air for many hours. They can cover enormous distances.

OK, this is the story on the part of the viruses. But what about the human side of the story? What is our defence against viruses?

Human have been living with viruses forever. 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 defences that have developed over time. Antibodies are just one of them. Against viruses with each a different survival strategy (Opportunists, Survivors and Airmen), we have developed a matching defence strategy.

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

Our oldest defences may have been developed against Opportunists. We have a skin. Our skin blocks almost everything harmful that our body wants to enter. If we have a wound, our body is very capable of healing that wound very quickly. But as long as that wound has not healed, Opportunists have the chance to enter our body.

And what is our defence strategy against Survivors (such as diarrhoea viruses)?

Viruses that want to enter our body through food, water or fingers: for them we have developed, among other things, a very sour stomach. Our gastric juice is a good barrier against most microorganisms: viruses are damaged as a result and 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: what about our defences against Flyers (such as flu viruses and corona)?

To get deep into the lungs, a Flyer must first pass through the mouth or nose. We all have several defence mechanisms in our nose, mouth and throat, including cells that mount an immune response. An entire army with a vanguard, a rear-guard 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 inactivated there. The mouth and pharynx also participate in the defence against the Flyers. If they get into the stomach, they don’t stand a chance there.

Does it matter whether a Flyer enters via a large or a small drop (aerosol)?

Yes. If a Flyer hitches a ride with a small drop (aerosol), the chance that he will break right through the defence line is much higher than with a large drop. Just as small fish are much more likely to slip through the mesh of a net than large fish. In addition, together with the inhaled air, the Flyer can fly deep into the lungs. After all, the drop is so small that – with a little luck – it can go straight into the lungs. The Flyer virus then hitches a ride with the aerosol or IS the aerosol when the water has evaporated. If a Flyer hitches a ride in a big drop, he has a much more difficult 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 very high.

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 defences 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 Flyers, this mainly has to do with the temperature and the humidity. Flyers 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 worldwide: there the air is kept cool and dry and in order to save costs, the outside air is insufficiently refreshed: cloud-9 for the Flyers (aerosols). In the winter we heat our houses and we are more indoors. The air dries out and we live together in a smaller space. In those conditions you can then inhale 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 barriers to intelligent systems that only supply fresh air when it is really needed), are usually at the expense of fresh and more humid outdoor air. 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 people outside?

Meeting outside is indeed sensible. You can compare the behaviour of aerosols with the behaviour 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 with you and you meet up with people outside, they will not be bothered by your aerosols. Except when you blow them in the face of course.

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 a few days later. 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 stop Flyers such as the corona virus?

Very important in the fight against all infectious diseases is: ensure that your natural defence system can function properly. Think of a healthy diet, sufficient exercise and enough sleep. Take an extra dose of vitamins C and D daily. These vitamins help to strengthen 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 up with friends, go out. And avoid “super spreading events” at all times.

What do you mean “super spreading events”? Tell me more about that…

The coronavirus can spread super-fast during so-called “super spreading events”. These are events where many people spend hours together in a poorly ventilated space. Think, for example, of Après-ski bars or carnival cafes. Only one person needs to carry the corona virus. That one person can  blow many aerosols into the air without noticing. 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 defences 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 diarrhoea viruses and many viruses that cause the common cold. Survivors travel from host to host through surfaces, through hands, 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 Flyer. And Flyers have a different strategy of traveling from host to host, and of invading hosts.

As said, Flyers mainly spread by air. Compare it again with cigarette smoke: it hardly spreads through hands or surfaces. Try blowing cigarette smoke on your hands, then give someone else 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. Flyers  need a lift of aerosols, in order to be able to get deep into the lungs where our body has less resistance. The measures to wash, disinfect, keep your distance, do not shake hands and do not hug are therefore not or hardly effective when you want to tackle Flyers such as flu viruses or corona.

But Menno, isn’t that diametrically opposed to what Mark Rutte (the Dutch Prime Minister) 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, 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 an enormous increase in the number of corona infections. Also here in Amsterdam East. How do you see that?

The infections are now measured with the so-called “PCR test”. This test examines whether you have a 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 throat are busy “getting to know them and standing by the front door”. As I just said: in 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 “infection” 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), and partly because we are testing more.

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

Yes and no. People who have made their acquaintance with the virus and managed to stop it 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. Problems can range from some cough to a febrile episode. In the Netherlands there is a great sense of togetherness, and the care for the elderly is wonderful. 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, immune to corona), the more favourable it is for our fellow human beings with a very weak immune system.

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

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 “excess-mortality”. 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 deadliness 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% or less in all age groups. This can be compared to the IFR in a medium to severe flu season. Yes, corona is a new virus. But I want to make a case for moving from fear to trust. We count other benign brothers and sisters of this coronavirus under our normal winter colds. We should not panic! People I have seen in the tropics generally deal more rationally with the much more severe infections.

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 poor even in rich societies are particularly vulnerable. This part of the population has an increased risk of serious complications, also because of other risk factors, such as obesity and diabetes. Bad food, bad air, bad housing, bad (drinking) water supplies all contribute directly or indirectly to a weakened immune system.

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

Face 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 disadvantages appear to outweigh the risk: breathlessness, concentration problems, headaches and psychological damage, particularly in young children.

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

I expect 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 improve ventilation in our buildings, then 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, and it could join the other corona winter colds.

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 hospital bed for economic reasons 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. That the savings from these cuts are not evenly distributed between people has resulted in a larger part of the population unable to follow a healthy lifestyle. As far as the approach is concerned, I think that a study of the older literature and targeted research based on it can contribute to an effective policy. A policy devoid of fear, panic and the need to sacrifice personal freedoms in a significant way.

Thank you very much for this interview, Menno!

Robijn Tilanus is an improvisation coach, pianist, composer and biologist. She writes as a freelance journalist for Oost-Online about topics that affect her: nature, music, health and striking people with a mission. Robijntilanus.nl