Steve Jones - The Language of the Genes

Patterns of infection depend on the number of potential victims. As a result, the importance of disease has changed throughout history. The longer an illness has a hold and the more efficiently it is transmitted, the smaller the population needed to allow it to persist. Immunity also plays a part. Some sicknesses must have started long before others. Signs of tuberculosis, which can drag on for decades, can be seen in the bones of those who died tens of thousands of years ago. In contrast, measles is new. It does not last long and is not very infectious. Those who have been infected become immune and cannot be attacked again so that a large population is needed to keep it going. The history of measles is close to those of the many novel ills that have inflicted themselves upon the human species since it began.

In a population which has never been exposed and which has no immunity, measles can have terrible effects. When it came to Fiji in 1875 (the result of a visit by the King to Sydney) it killed a third of his hundred and fifty thousand subjects. It soon disappeared from the island as it needs a community of at least half a million to persist. Measles may arrive in a place with fewer inhabitants, but cannot maintain itself. In Iceland before the Second World War there were gaps of up to seven years between epidemics. Only after 1945 (when constant movement meant that the Icelanders became part of the European population as a whole) did measles become a continuous problem. Humans have lived in groups of half a million or more for a mere two or three thousand years, so that measles must be a fairly new disease. Its initial impact was much worse than was its effect on populations who had lived with ii lor many generations.

The constant change in the pattern of infection means that evolution can never rest: far from perfecting us, it is constantly faced with new challenges. Ten thousand years ago, when humans lived in small bands, contagious disease may scarcely have existed. No doubt there were plenty of lice and tapeworms around as their long lives and ability to reinfect their own hosts mean that they do not need many people to keep going. In general the ancient world was a healthy one. People starved, froze or were eaten by tigers instead. Even when an epidemic struck, it was a local matter. The few hunter-gatherers who remain show remnants of this pattern. In the 1950s, tribal groups of Yanomamo differed greatly in the antibodies that they possessed. In some villages, everyone had antibodies to (and must have been infected with) chickenpox. In others, that disease had never arrived but the whole population had once had influenza. The pattern of disease reflected a balance between a chance arrival of a new pathogen and a local epidemic which ended as soon as everyone was immune or dead. The same pattern exists in chimpanzees today. Now, the Yanomamo have joined the rest of humanity and its diseases and have suffered as a result.

With farming, the human population shot up and began to coalesce into one continental mass. A whole new set of disorders appeared. Irrigation helped with the appearance of water-borne parasites such as schistosomes, which are carried by snails. Their eggs have been found in mummies from 1200 BC. Schistosomiasis is still common in Egypt. Many of the biblical plagues were new diseases that took hold as the population of Egypt grew large enough to sustain them.

Some contagions came from animals. The closest relative of measles is the cattle disease rinderpest and measles itself may have evolved from this. A relation of smallpox is found in cows and of sleeping sickness in wild game. Only a small genetic change in the parasites was required to allow them to attack a new host. Homo sapiens.

Some maladies came and went and have never been identified. The towns of mediaeval Europe suffered from outbreaks of dancing mania in which thousands took part. Some may have been due to mass hysteria, but the swellings and pain suggest an organic cause. In Italy the affliction was called tarantism and ascribed (wrongly) to the bite of spiders. St Vitus' Dance, with its visions of God, may have been the same illness. The epidemics began in eleventh-century Germany and had disappeared by the seventeenth century. England had its own mysterious and transient illness, the English Sweat, which came and went several times between 1480 and 1550.

The effects were terrifying. It was brought into London by soldiers fleeing the Battle of Bosworth and a month later was at its peak. It killed its victims within a day. The University of Oxford was closed for six weeks. The disease returned several times over the next fifty years and in parts of Europe the mortality was so high that eight corpses were put into each grave. The last epidemic started in Shrewsbury in 1551, killing thousands. Since then the disease has gone. What it was, nobody knows.

Hippocrates, in the fourth century before Christ, was the first to describe symptoms well enough to allow diseases to be diagnosed with any confidence. Ancient Greece had diphtheria, tuberculosis and influenza, but none of his records suggest the presence of smallpox, bubonic plague or measles. Travel soon led to a new set of pestilences. Smallpox was in India a thousand years before Christ, but its'short incubation period meant that it killed its carriersquickly and did not travel well by land. It reached Europe by sea, with the first epidemic in Rome in 165 BC. Perhaps the disease helped in the spread of Christianity, as even to give a sufferer a glass of water can relieve some of the symptoms. Anglo-Saxon records mention nearly fifty epidemics between 516 and 1087.

Life got even worse in the first large towns. Big cities are recent things. Before 1800 just one European in fifty lived in a settlement of more than a hundred thousand people. There has been movement from the countryside for a thousand years, but epidemics meant that no city could sustain its own numbers until the nineteenth century. London at the time of Pepys had a population of a hundred thousand, but it needed five thousand immigrants a year to maintain its population in the face of pestilence.

Plague had killed millions in the centuries before his day, but its last and worst epidemic was during Pepys' own lifetime. In December 1664, two Frenchmen died in Drury Lane. In the following June, Pepys wrote in his diary: 'This day, much against my will, I did in Drury Lane see two or three houses marked with a red cross upon the door, and the "Lord have Mercy upon Us" writ there: which was a sad sight to me, being the first of the kind that, to my remembrance, I ever saw. It put me into an ill conception of myself and my smell, so that I was forced to buy some roll-tobacco to smell and to chew, which took away my apprehension/ By the summer of that year, two thirds of the population of London had fled and the disease raged throughout England. The cycle of epidemics which tormented the capital and reached its peak in the Plague Year of 1665 ended with the replacement of thatched roofs (and their resident rats) by slates after the Great Fire in 1666. The last European plague was a century later, in the Balkans. The disease has often been introduced since then, but has never spread. Three hundred years ago, England had great cycles of death. Life expectancy fell from forty-two years in the late sixteenth century to thirty years in the seventeenth, returning to the earlier level only in Victorian times. The greatest mortality was in low-lying villages. 'Fevers* were usually blamed. City conscripts impressed into the army did better than the healthier youth from the countryside. The urban soldiery were pinched and weak, bur had been exposed to infection so often that they were immune to the diseases which slaughtered their country cousins when forced into crowded barracks.

New contagions continue to appear. As well as AIDS, Africa had another mysterious epidemic in the 1970s, when outbreaks of the deadly (and until then unknown) Ebola fever killed half those infected. Even a trivial change can spark off new illnesses. In the past thirty years, Lyme Disease (named after the village of Lyme, in Connecticut, where it first appeared) has become the most widespread pest-borne disease in the United States, with more than ten thousand cases a year. It causes arthritis and a variety of painful nervous symptoms. The malady is due to a microorganism which spends part of its time in a tick found on white-tailed deer. A few cases were known a century ago but it did not become common until people moved to the suburbs and were exposed to the deer that flourish there. Nineteenth-century sanitation meant that cities became safer places. But it had a cost. Before sewers, every infant was exposed to a constant small dose of polio virus. Their immune system works well and most became resistant. Once the water had been cleaned up only those few children unlucky enough to come into contact with a sudden dose of the virus got the disease. If the World Health Organisation succeeds (as it hopes to) in eliminating it altogether, any accidental escape might cause a disaster.