The Greatest Benefit to Mankind: A Medical History of Humanity. Roy Porter
courts and kingdoms. But while agriculture rescued people from starvation, it unleashed a fresh danger: disease.
The agricultural revolution ensured human domination of planet earth: the wilderness was made fertile, the forests became fields, wild beasts were tamed or kept at bay; but pressure on resources presaged the disequilibrium between production and reproduction that provoked later Malthusian crises, as well as leading to ecological deterioration. As hunters and gatherers became shepherds and farmers, the seeds of disease were sown. Prolific pathogens once exclusive to animals were transferred to swineherds and goatherds, ploughmen and horsemen, initiating the ceaseless evolutionary adaptations which have led to a current situation in which humans share no fewer than sixty-five micro-organic diseases with dogs (supposedly man’s best friend), and only slightly fewer with cattle, sheep, goats, pigs, horses and poultry.
Many of the worst human diseases were created by proximity to animals. Cattle provided the pathogen pool with tuberculosis and viral poxes like smallpox. Pigs and ducks gave humans their influenzas, while horses brought rhinoviruses and hence the common cold. Measles, which still kills a million children a year, is the result of rinderpest (canine distemper) jumping between dogs or cattle and humans. Moreover, cats, dogs, ducks, hens, mice, rats and reptiles carry bacteria like Salmonella, leading to often fatal human infections; water polluted with animal faeces also spreads polio, cholera, typhoid and viral hepatitis.
Settlement helped disease to settle in, attracting disease-spreading insects, while worms took up residence within the human body. Parasitologists and palaeopathologists have shown how the parasitic roundworm Ascaris, a nematode growing to over a foot long, evolved in humans, probably from pig ascarids, producing diarrhoea and malnutrition. Other helminths or wormlike fellow-travellers became common in the human gut, including the Enterobius (pinworm or threadworm), the yards-long tapeworm, and the filarial worms which cause elephantiasis and African river blindness. Diseases also established themselves where agriculture depended upon irrigation – in Mesopotamia, Egypt, India and around the Yellow (Huang) River in China. Paddyfields harbour parasites able to penetrate the skin and enter the bloodstream of barefoot workers, including the forked-tailed blood fluke Schistosoma which utilizes aquatic snails as a host and causes bilharzia or schistosomiasis (graphically known as ‘big belly’), provoking mental and physical deterioration through the chronic irritation caused by the worm. Investigation of Egyptian mummies has revealed calcified eggs in liver and kidney tissues, proving the presence of schistosomiasis in ancient Egypt. (Mummies tell us much more about the diseases from which Egyptians suffered; these included gallstones, bladder and kidney stones, mastoiditis and numerous eye diseases, and many skeletons show evidence of rheumatoid arthritis.) In short, permanent settlement afforded golden opportunities for insects, vermin and parasites, while food stored in granaries became infested with insects, bacteria, fungoid toxins and rodent excrement. The scales of health tipped unfavourably, with infections worsening and human vitality declining.*
Moreover, though agriculture enabled more mouths to be fed, it meant undue reliance on starchy cereal monocultures like maize, high in calories but low in proteins, vitamins and minerals; reduced nutritional levels allowed deficiency diseases like pellagra, marasmus, kwashiorkor and scurvy to make their entry onto the human stage. Stunted people are more vulnerable to infections, and it is a striking comment on ‘progress’ that neolithic skeletons are typically some inches shorter than their palaeolithic precursors.
MALARIA
Settlement also brought malaria. ‘There is no doubt’, judged the distinguished Australian immunologist, Macfarlane Burnet (1899–1985), ‘that malaria has caused the greatest harm to the greatest number’ – not through cataclysms, as with bubonic plague, but through its continual winnowing effect. First in sub-Saharan Africa and elsewhere since, conversion of forests into farmland has created environments tailormade for mosquitoes: warm waterholes, furrows and puddles ideal for rapid breeding. Malaria is worth pausing over, since it has coexisted with humans for thousands of years and remains out of control across much of the globe.
The symptoms of malarial fevers were familiar to the Greeks, but were not explained until the advent of tropical medicine around 1900. They are produced by the microscopic protozoan parasite Plasmodium, which lives within the body of an Anopheles mosquito, and is transmitted to humans through mosquito bites. The parasites move through the bloodstream to the liver, where they breed during an incubation stage of a couple of weeks. Returning to the blood, they attack red blood cells, which break down, leading to waves of violent chills and high fever.
Malarial parasites have distinct periodicities. Plasmodium vivax, the organism causing benign tertian malaria, once present in the English fenlands, has an incubation period of ten to seventeen days. The fever lasts from two to six hours, returning every third day (hence ‘tertian’); marked by vomiting and diarrhoea, such attacks may recur for two months or longer. In time, as Greek doctors observed, the spleen enlarges, and the patient becomes anaemic and sometimes jaundiced. Quartan malaria, caused by Plasmodium malariae, is another mild variety.
Malignant tertian malaria, caused by Plasmodium falciparum, is the most lethal, producing at least 95 per cent of all malarial deaths. The incubation period is shorter but the fever more prolonged; it may be continuous, remittent or intermittent. Plasmodium falciparum proliferates fast, producing massive destruction of red blood cells and hence dangerous anaemia; the liver and spleen also become enlarged.
Malaria may sometimes appear as quotidian fever, with attacks lasting six to twelve hours – the result of multiple infection. Patients may also develop malarial cachexia, with yellowing of the skin and severe spleen and liver enlargement; autopsy shows both organs darkened with a black pigment derived from the haemoglobin of the destroyed red blood cells. What the ancients called melancholy may have been a malarial condition.
Malaria shadowed agricultural settlements. From Africa, it became established in the Near and Middle East and the Mediterranean littoral. The huge attention Graeco-Roman medicine paid to ‘remittent fevers’ shows how seriously the region was affected, and some historians maintain the disease played its part in the decline and fall of the Roman empire. Within living memory, malaria remained serious in the Roman Campagna and the Pontine marshes along Italy’s west coast.
Coastal Africa was and remains heavily malarial, as are the Congo, the Niger and hundreds of other river basins. Indigenous West African populations developed a genetically controlled characteristic, the ‘sickle-cell’, which conferred immunity against virulent Plasmodium falciparum. But, though protective, this starves its bearers, who are prone to debility and premature death: typical of such evolutionary trade-offs, gains and losses are finely balanced.
India was also ripe for malarial infection. Ayurvedic medical texts (see Chapter Six) confirm the antiquity of the disease in the subcontinent. China, too, became heavily infected, especially the coastal strip from Shanghai to Macao. And from the sixteenth century Europeans shipped it to Mesoamerica: vivax malaria went to the New World in the blood of the Spanish conquistadores, while falciparum malaria arrived with the African slaves whom the Europeans imported to replace the natives they and their pestilences had wiped out.
Malaria was just one health threat among many which set in with civilization as vermin learned to cohabit with humans, insects spread gastroenteric disorders, and contact with rodents led to human rickettsial (lice-, mite- and tick-borne) arbo diseases like typhus. Despite such infections encouraged by dense settlement and its waste and dirt, man’s restless inventive energies ensured that communities, no matter how unhealthy, bred rising populations; and more humans spawned more diseases in upward spirals, temporarily and locally checked but never terminated. Around 10,000 BC, before agriculture, the globe’s human population may have been around 5 million; by 500 BC it had probably leapt to 100 million; by the second century AD that may have doubled; the 1990 figure was some 5,292 million, with projections suggesting 12 billion by 2100.
Growing numbers led to meagre diets, the weak and poor inevitably bearing the brunt. But though humans were often malnourished, parasite-riddled and pestilence-smitten,