'Although the Soldiery retreated from the Field of Death, and encamped out of the City, the Contagion followed, and vanquish'd them; many in their Old Age, and others in their Prime, sunk under its cruelties; of the Female Sex most died; and hardly any children escaped; and it was not uncommon to see an Inheritance pass successively to three or four Heirs in as many Days; the Number of Sextons were not sufficient to bury the Dead.'

—Nathanial Hodges; Loimologia: an account of the 1665 London Plague

Throughout history, humanity has fallen victim to pandemics of cholera, plague, influenza, typhoid, tuberculosis and other infectious maladies so widespread, most people rarely made it into middle age. Other (seemingly more banal ailments) such as ear, skin and throat infections often resulted in deafness, disfigurement and/or death due to blood poisoning and other complications.

As recently as the 19th century, the average life span in Europe and North America was around 50 years and marked by the steady – if predictable – loss of family, friends, spouses and colleagues. The habit of enquiring after another's health was a meaningful and entrenched nicety based on the ever-present threat of sudden death due to pestilence, accidents or random infection caused by any number of hostile pathogens. It was a world in which the likelihood of dying prematurely from infectious diseases was as high as 40%, and where women routinely succumbed during childbirth to infections easily curable by today's standards.

In developing nations the situation was even worse – with one caveat; unlike industrialized nations, conditions in developing nations never really improved. In poorer nations today infectious diseases – both major and seemingly minor – further contribute to premature death and the ongoing misery of underprivileged populations.

Helpless Against Diseases

For previous generations living in industrialized nations, frequent illness replete with fears of sudden (or slow) death marred a life that was already "nasty, brutish and short". Such an existence could well have been the miserable consequence of being born in the first place. Few people escaped debilitating disease, disfiguring skin conditions or the likelihood of pain and suffering caused by any number of cunning microbes. Life was transient, ephemeral and characterized by an endless cycle of grief and loss.

In Europe, wave after wave of epidemics kept humanity forever teetering on the edge of demographic collapse. Between the 14th and 15th centuries, the continent saw its population halved by successive outbreaks of smallpox, typhus, and the ever-present menace of the Black Death (plague). In India, plague didn't appear to take hold until 1896 when a strain appeared out of Yunnan in China. By 1903 more than 1.3 million Indians were dying every year. Overall, historians estimate that between 1896 and 1948 12.5 million people died of plague on the subcontinent alone. In 1783, British historians calculated that some 20 000 pilgrims to the Indian holy site of Hardawar succumbed to cholera. Within months the bacilli spread outwards towards China, north to Russia and southwest to the Middle East. By 1831, cholera infected nearly half of the Haj faithful making the annual pilgrimage to sacred sites in Mecca and Medina – the fatal consequence of drinking from a single ritualistic source of contaminated water. Dehydrated and shedding vibrios, dying pilgrims crept homewards depositing bacteria along key transportation routes. The great ports of Alexandria and Istanbul were soon staggering under a cholera epidemic that subsequently radiated outwards throughout the entire North African littoral, into the Balkans, up the Danube and onwards towards Hungary leaving behind a trail of corpses, orphans, economic ruin and contaminated water and food.

In the early 1800s, outbreaks of puerperal sepsis – a streptococcal infection – were responsible for the deaths of upwards of 70% of new mothers "lying in" at hundreds of small hospitals flecking Europe. In one Italian infirmary it was reported that not a single woman survived childbirth over an entire year. Not until health workers (who routinely moved between morgue and maternity room) adopted hand-washing in the late 1800s, did the death rate decline.

The end of the First World War saw seemingly innocuous ailments such as the ubiquitous "touch of grippe" metamorphose into grim harbingers of contagion and death. In the fall of 1918, a tidal wave of influenza rolled over Europe, Asia, Australia, North and South America killing millions and devastating entire economies. A viral infection, influenza today rarely kills on its own but erodes the lining of the respiratory tract thereby allowing secondary "super" infections (often bacterial) to take hold.

British demographer Kingsley David once suggested that influenza wiped out 20 million in India, while contemporary experts tag the total number of dead worldwide at 30 million – more than those killed in the Great War itself. Among the world's aboriginal peoples – most notably the Inuit of northern Canada – the epidemic exacted a harsh and bitter toll. Formerly isolated from the disease, such populations suffered the most.

In Samoa, 25% of the islands' people died, while in Canada entire Inuit villages sickened and then fell silent under the deathly pall of influenza-related infections.

A Destroyer of Empires

It is arguable whether war or the devastation wrought by infectious disease has had a greater historic influence on political boundaries. Up until the Second World War, it was pestilence – and not warfare – that claimed the lives of Europe's soldiers. Napoleon Bonaparte can lay blame for his ignominious retreat from Moscow – not on the Russians, nor even the Russian winter. By far, his deadliest opponent was typhus; a louse-borne infection that reduced a healthy Grande Armee of 655 000 to a pitiful and demoralized 93 000 – who wound up straggling home and surviving just long enough to pass the rickettsia on to neighbours and loved ones. The subsequent epidemic killed another two million, carrying off 250 000 civilians in Germany alone.

In the New World, it was not superior Spanish firepower, nor their reliance on horses that resulted in the conquest and enslavement of the Amerindians. By far the greatest allies of the self-proclaimed, "liberators of the heathens" were smallpox, influenza and measles. Formerly unknown in the Americas, the first recorded smallpox epidemic hit the fledgling colony of Santo Domingo in 1495, destroying 80% of the local indigenous population. That same outbreak was also responsible for the deaths of hundreds of Spanish soldiers after the battle of Vega Real in 1495.

In 1515, another flare-up in Puerto Rico spared the Spanish but extirpated the locals. By the time Hernando Cortes and his rogue's army of mercenaries and missionaries set foot on Mexico's shores, smallpox, measles and influenza had already insinuated themselves as a kind of microbial fifth column among the local population. How a ragtag army of 300 men (albeit armed with muskets, riding horses and unbridled greed) could defeat the highly organized and warlike Aztecs can never be satisfactorily explained except by factoring in the inroads European diseases made into a people entirely devoid of immunity. Conquistador and expedition scribe Bernal Diaz described the resultant carnage from infectious disease thus: "We could not walk without treading on the bodies and heads of dead Indians. The dry land was piled with corpses." In the space of 10 years, historians estimate that Mexico's population plummeted from some 25 million to 6.5 million owing to epidemics of infectious disease – a drop of 74%. In North America, later events echoed those in Mexico but with one not-so-subtle difference. By the 1600s, colonizers knew enough about epidemiology to maliciously inflict deadly diseases on locals by providing "gifts" of blankets and clothing infested with smallpox and typhus-bearing lice – the first recorded acts of biological warfare.

Terrible Old Visitors Who Won't Go Away

Today's most virulent killers have been at work for centuries. Malaria and acute respiratory infections have killed multitudes throughout much of human history. Indeed, forensic archaeologists discovered TB bacteria hiding out in the tissue of mummies thousands of years old.

If smallpox was Europe's primary export to the New World, the New World may well have hit back with syphilis. After engaging in rape and pillage, Spanish conquistadors, troops and camp-followers sailed home to Europe only to scatter the seeds of yet another epidemic. This new sexually transmitted contagion was characterized by genital ulcers that progressed to rash, dementia and hideous abscesses that gnawed away at flesh and bone. Henry VIII, Sir Randolph Churchill (Winston's father), Schopenhauer and Guy de Maupassant were just a few who may have met an ugly end courtesy of the peripatetic spirochaete.

We're Vulnerable Without Effective Medicines

Most people alive in industrialized nations today know a grandmother or great uncle who can describe how whooping cough, influenza or diphtheria whittled away friends and family. In previous epochs – and still today in many developing nations – a simple bladder infection could lead to death by kidney failure; minor skin conditions such as impetigo could end in scarring and lifelong disfigurement; and killers such as measles, tuberculosis and pneumonia stalked uncontested though the streets, offices and homes of every city and hamlet around the world.

Today, the situation in developing nations remains as grim as that of previous generations in industrialized nations. In impoverished regions of the world, almost everyone knows a relative who suffers from infectious diseases of poverty such as simple diarrhoea, respiratory infections, tuberculosis, malaria or AIDS.

In the developing world – where poverty and inadequate access to healthcare remain oppressive reminders of human frailty – infectious disease continues to be an omnipresent threat to life and livelihood. Owing to the absence of hard historical data, one can only extrapolate former conditions of overcrowding and poor health once prevailing in Europe to developing nations today.