10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness. Alanna Collen
She had been infected by a strain of smallpox known as ‘Abid’, named after a three-year-old Pakistani boy who had succumbed to the disease eight years previously, shortly after the WHO’s intensive smallpox eradication campaign had got under way in Pakistan. Smallpox had become a significant killer across most of the world by the sixteenth century, in large part due to the tendency of Europeans to explore and colonise other regions of the world. In the eighteenth century, as human populations grew and became increasingly mobile, smallpox spread to become one of the major causes of death around the world, killing as many as 400,000 Europeans each year, including roughly one in ten infants. With the uptake of variolation – a crude and risky predecessor of vaccination, involving intentional infection of the healthy with the smallpox fluids of sufferers – the death toll was reduced in the latter half of the eighteenth century. Jenner’s discovery of vaccination using cowpox in 1796 brought further relief. By the 1950s, smallpox had been all but eliminated from industrialised countries, but there were still 50 million cases annually worldwide resulting in over 2 million deaths each year.
Though smallpox had released its grip on countries in the industrialised world, the tyrannical reign of many other microbes continued in the opening decade of the twentieth century. Infectious disease was by far the dominant form of illness, its spread aided by our human habits of socialising and exploring. The exponentially rising human population, and with that, ever-greater population densities, only eased the person-to-person leap that microbes needed to make in order to continue their life cycle. In the United States, the top three causes of death in 1900 were not heart disease, cancer and stroke, as they are today, but infectious diseases, caused by microbes passed between people. Between them, pneumonia, tuberculosis and infectious diarrhoea ended the lives of one-third of people.
Once regarded as ‘the captain of the men of death’, pneumonia begins as a cough. It creeps down into the lungs, stifling breathing and bringing on a fever. More a description of symptoms than a disease with a sole cause, pneumonia owes its existence to the full spectrum of microbes, from tiny viruses, through bacteria and fungi, to protozoan (‘earliest-animal’) parasites. Infectious diarrhoea, too, can be blamed on each variety of microbe. Its incarnations include the ‘blue death’ – cholera – which is caused by a bacterium; the ‘bloody flux’ – dysentery – which is usually thanks to parasitic amoebae; and ‘beaver fever’ – giardiasis, again from a parasite. The third great killer, tuberculosis, affects the lungs like pneumonia, but its source is more specific: an infection by a small selection of bacteria belonging to the genus Mycobacterium.
A whole host of other infectious diseases have also left their mark, both literally and figuratively, on our species: polio, typhoid, measles, syphilis, diphtheria, scarlet fever, whooping cough and various forms of flu, among many others. Polio, caused by a virus that can infect the central nervous system and destroy nerves controlling movements, paralysed hundreds of thousands of children each year in industrialised countries at the beginning of the twentieth century. Syphilis – the sexually transmitted bacterial disease – is said to have affected 15 per cent of the population of Europe at some point in their lifetime. Measles killed around a million people a year. Diphtheria – who remembers this heart-breaker? – used to kill 15,000 children each year in the United States alone. The flu killed between five and ten times as many people in the two years following the First World War than were killed fighting in the war itself.
Not surprisingly these scourges had a major influence on human life expectancy. Back then, in 1900, the average life expectancy across the whole planet was just thirty-one years. Living in a developed country improved the outlook, but only to just shy of fifty years. For most of our evolutionary history, we humans have managed to live to only twenty or thirty years old, though the average life expectancy would have been much lower. In one single century, and in no small part because of developments in one single decade – the antibiotic revolution of the 1940s – our average time on Earth was doubled. In 2005, the average human could expect to live to sixty-six, with those in the richest countries reaching, again on average, the grand old age of eighty.
These figures are highly influenced by the chances of surviving infancy. In 1900, when up to three in ten children died before the age of five, average life expectancy was dramatically lower. If, at the turn of the next century, rates of infant mortality had remained at the level they were in 1900, over half a million children would have died before their first birthday in the United States each year. Instead, around 28,000 did. Getting the vast majority of children through their first five years unscathed allows most of them to go on and live to ‘old age’ and brings the average life expectancy up accordingly.
Though the effects are far from fully felt in much of the developing world, we have, as a species, gone a long way towards conquering our oldest and greatest enemy: the pathogen. Pathogens – disease-causing microbes – thrive in the unsanitary conditions created by humans living en masse. The more of us we cram onto our planet, the easier it becomes for pathogens to make a living. By migrating, we give them access to yet more humans, and in turn, more opportunity to breed, mutate and evolve. Many of the infectious diseases we have contended with in the last few centuries originated in the period after early humans had left Africa and set up home across the rest of the world. Pathogens’ world domination mirrored our own; few species have as loyal a pathogenic following as us.
For many of us living in more developed countries, the reign of infectious diseases is confined to the past. Just about all that remain of thousands of years of mortal combat with microbes are memories of the sharp prick of our childhood immunisations followed by the ’reward’ of a polio-vaccine-infused sugar lump, and perhaps more clearly, the melodramatic queues outside the dinner hall as we waited with our school friends for a teenage booster shot. For many children and teenagers growing up now, the burden of history is even lighter, as not only the diseases themselves, but once-routine vaccinations, such as the dreaded ‘BCG’ for tuberculosis, are no longer necessary.
Medical innovations and public health measures – largely those of the late nineteenth and early twentieth centuries – have made a profound difference to life as a human. Four developments in particular have taken us from a two-generation society to a four-, or even five-generation society in just one, long, lifetime. The first and earliest of these, courtesy of Edward Jenner and a cow named Blossom, is, of course, vaccination. Jenner knew that milkmaids were protected from developing smallpox by virtue of having been infected by the much milder cowpox. He thought it possible that the pus from a milkmaid’s pustules might, if injected into another person, transfer that protection. His first guinea pig was an eight-year-old boy named James Phipps – the son of Jenner’s gardener. Having inoculated Phipps, Jenner went on to attempt to infect the brave lad, twice injecting pus from a true smallpox infection. The young boy was utterly immune.
Beginning with smallpox in 1796, and progressing to rabies, typhoid, cholera and plague in the nineteenth century, and dozens of other infectious diseases since 1900, vaccination has not only protected millions from suffering and death, but has even led to countrywide elimination or complete global eradication of some pathogens. Thanks to vaccination, we no longer have to rely solely on our immune systems’ experiences of full-blown disease to defend us against pathogens. Instead of acquiring natural defences against diseases, we have circumvented this process using our intellect to provide the immune system with forewarning of what it might encounter.
Without vaccination, the invasion of a new pathogen prompts sickness and possibly death. The immune system, as well as tackling the invading microbe, produces molecules called antibodies. If the person survives, these antibodies form a specialist team of spies that patrol the body looking out specifically for that microbe. They linger long after the disease has been conquered, primed to let the immune system know the moment there is a reinvasion of the same pathogen. The next time it is encountered, the immune system is ready, and the disease can be prevented from taking hold.
Vaccination mimics this natural process, teaching the immune system to recognise a particular pathogen. Instead of suffering the disease to achieve immunity, now we suffer only the injection, or oral administration, of a killed, weakened or partial version of the pathogen. We are spared illness but our immune systems still respond to the introduction of the vaccine, and produce antibodies that help the body to resist disease if the same pathogen invades for real.