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It is possible to capture the liquid in your breath by cooling it down – a bit like when you breathe on a cold mirror. If you do this, over one minute you can recover 100 microlitres of fluid (about the size of a lentil). If you then grow what you recover from that fluid you get between 100 and

10,000 bacteria, 5,000 viruses and one or two fungi. Every time we breathe, we inhale a cocktail of potential pathogens. And we inhale about 20 times a minute, nearly 30,000 times a day, sucking in 15,000 litres of air.

The world through which we move is awash with potential pathogens. They are not just in the air we breathe, they are in the water we drink, on the lettuce we eat and in the soil on which we stand. And we don’t just find microorganisms in the external environment; they are on and in us. Our skin plays host to thousands of bugs and our guts are packed full of bacteria – by some ways of counting there are more cells of bacteria in your body than there are cells of you (more of which later). We very much live in a microbial world. Most of them either ignore our existence or live side by side in perfect harmony. But all of the pathogenic microorganisms – the infections – are just waiting for an in.

Most remarkably, it can take fewer than ten actual viruses to infect you. A virus is 100 nm in diameter, which means that in a sugar cube made of virus there would be about 1,000,000,000,000,000 viruses, enough to infect every single person on the planet one hundred thousand times. It was estimated that if you had collected all of the SARS viruses from all of the infected people at the peak of the pandemic, they would not even have filled a Coke can.

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Which leads us to the big question: why don’t we get sick all the time?

There are two elements to this: the environment and host defence. The environment determines whether you encounter the pathogen in the first place: host defence encompasses how your body protects you from the pathogen if you do encounter it.

It might seem obvious, but you can’t get infected with a pathogen if you never come across it. None of us will ever get smallpox, because of its complete eradication from the globe. There are regional variations in pathogen exposure. and these can be determined by a range of factors. For example, you cannot catch a vector-borne pathogen if there are no vectors where you live, so you cannot catch malaria in the UK because the Anopheles mosquito is no longer native to the UK. Other pathogens are limited to certain countries, either due to deliberate efforts to eradicate them or because of interventions to reduce their prevalence. Clean water removes the likelihood of exposure to a range of gut (also called enteric) pathogens such as cholera and typhoid.

The weather also has an impact: for reasons we don’t completely understand, some pathogens are much more common in the winter than in the summer. In warmer temperatures the droplets evaporate quicker, increasing the concentration of chemicals in the drop, like spilled Coca-Cola getting stickier if left. This increase in concentration alters the acidity of the droplet, which in turn affects how the virus gets into our cells. In the cooler winter months, the droplets that spread the virus are more stable. The amount of sunlight also contributes to seasonality. The sun can protect us directly, its UV rays killing viruses, and also indirectly, as more sunlight means more vitamin D, which plays an important role in our immune response. Changes in social behaviour also contribute to seasonality. In winter we are more likely to be inside; especially children, who are a key vector for viral spread. Parents of nursery age children will be painfully aware of this. The winter flu season is only a feature of temperate climates – countries in the tropics don’t have a winter flu season, because they don’t have winter.

Frequency of exposure is also important. If there is a set risk of getting a disease each time you encounter it, then the more times you are exposed to it the more likely you are to catch it. If you do something often enough, rare events will eventually happen. The risk of infection can be quantified as a function of the number of contacts between an infected person and a susceptible person, the length of time they spend together and the likelihood of transmission each time they make contact.

Of course, exposure isn’t everything. Some people can be repeatedly exposed to a pathogen and never get infected; for example, a group of around 140 sex workers in Nairobi remained uninfected with HIV despite frequent exposure to the virus. Critically, there is a difference between infection and disease. The outcome of what happens once a pathogen gets into the body will depend upon the person infected and is determined by a combination of dose, genes, sex, age, behaviour and underlying conditions.

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Fundamentally, the pathogen must enter the body to cause an infection. This most frequently happens at the interfaces between us and our environment: the lungs, the guts, the genito-urinary tract and the skin. Transmission can happen directly through contacts (e.g. kissing) and droplets (e.g. sneezing) or indirectly from a contaminated surface (beware the infected lettuce leaf) or via a vector (mosquitoes, again).

Luckily, we possess a whole arsenal of host defences that stop pathogens getting into our bodies. Understanding these defences and how we can augment them underpins our successes in controlling pathogens, particularly through vaccination. As a card-carrying immunologist, I would like to claim that the sole reason we don’t get infections is because of our immune system. This would be entirely out of self-interest: the more important your subject sounds, the more money you can wring from the government! But the immune system is really the last resort – activating it comes with some risk of damage to your body. Many non-communicable diseases are caused by your immune system going rogue – including allergy, arthritis, asthma, Crohn’s, diabetes, Lupus and multiple sclerosis. These are often described as autoimmune conditions, because the body attacks itself. There is a Yin and Yang balance: not enough immunity get infections; too much immunity get arthritis.

Beyond the physical and mechanical mechanisms for preventing infection, our bodies are also masters of chemical warfare against the invading hordes. As you’ll know if you’ve ever tasted sick (note the disgust reaction), the stomach contents are highly acidic. This acid bath very effectively kills infectious bacteria in the stomach. Antacids, commonly taken to prevent heartburn, work by reducing the acidity in the stomach, but with a side effect of increased stomach infections. Reducing the acid in the stomach makes it a more hospitable environment for the bacteria. Another common chemical agent that prevents infection is mucus – or snot. And we don’t just produce mucus in our noses – it lines our lungs, our guts and our reproductive tracts. Mucus comprises a complex mixture of sugars, proteins and DNA. The proteins that make it sticky are called mucins. Our bodies make diff erent types of mucus at different sites: airway mucus needs to be thinner and more penetrable than gut mucus because of the difference in function between the lungs and the guts.

Mucus is just one part of a formidable stockpile of antimicrobial chemicals. We also use enzymes to destroy pathogens. Enzymes are proteins that speed up chemical reactions in the body. German physiologist Wilhelm Kühne named enzymes after the Greek word meaning yeast, because yeast accelerated the fermentation of fruit into alcohol. The enzymes that protect us against infection are mostly proteolytic; that is, they chew up other proteins. In an efficient twist, the enzymes that digest our food are also effective at breaking down the components of bacteria in the stomach. As well as the multifunctional digestive enzymes, we make enzymes that specifically target bacterial biochemicals.

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As the Martians discovered in The War of the Worlds, it’s a microbial war out there. In the same way that lions compete for prey on the plains of the Serengeti, microorganisms compete for nutrients and space in your body. And believe me, it is a fierce competition. Estimates vary but the numbers of things living on and in our bodies are mind-blowing. Until recently, it was widely stated that your body has ten times more cells of bacteria than it has cells of you. The ratio is probably closer to 1:1, depending on whether you measure the bacterial number before or after someone visits the toilet. A recent recalculation suggests that we are made of about 3 x 1013 (3 with 13 zeros) human cells, most of which are blood cells, and we carry 3.8 x 1013 bacteria cells, most of which live in the guts. But 38,000,000,000,000 is still a lot of bacteria – about 0.2 kg in fact. For the bacteria you are basically a walking buffet.

This excerpt from Dr. John S. Tregoning’s ‘Infectious’ has been published with permission from Simon & Schuster.

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