infectious diseases

infectious diseases are the result of damaging microorganisms obtaining access to the body, and not being repelled or destroyed by the immune system. Their relationship to man is that of parasite and host, and is continually adjusting. Numerous different types of bacteria, viruses, and other organisms may cause disease, and infection may take place through close contact with an infected person, or through the respiratory, digestive, or genito-urinary systems, depending on the organism and the disease involved. Infection may also occur by transmission from an animal, or via an insect vector. Organisms can damage the body by their multiplication in or around its cells, or by the widespread poisonous effects of substances (toxins) which they release. Many infectious diseases are of a self-limiting character, ending in either full recovery or death. While certain of them may occasionally have long-term sequelae, the body, if its defences win, for the most part returns to normal, often retaining a lifelong subsequent immunity against that specific infection. Other infections, such as syphilis, are, however, chronic, and eventually fatal if untreated.

Diseases included in the category ‘infectious’ include colds and influenza; the familiar infectious illnesses of childhood, and the more serious conditions such as poliomyelitis, diphtheria and meningitis, typhoid, typhus, cholera, dysentery, and smallpox. Tuberculosis is also an infectious disease, although its clinical progress is chronic rather than acute.

Most of these diseases have a very ancient history. While many only emerge as identifiable entities in the medical writings of the seventeenth and eighteenth centuries, others can be demonstrated to have been present in antiquity. Smallpox, for example, which was declared eradicated by the World Health Organisation in 1977, can clearly be identified by characteristic lesions on the mummified corpses of ancient Egyptians, while a stele of the same civilization, dating from 1580–1350 bc, shows a young man displaying a withered and shortened left leg, held in the ‘equinus position’ characteristic of paralysis possibly caused by poliomyelitis. Infectious diseases also occur in the animal kingdom, and some, such as anthrax and yellow fever, are transmissible to man.

Understanding

While the closely allied concepts of infection and contagion (transmission of disease from one person to another by direct or indirect contact) are probably almost as old as mankind, it was only in the mid nineteenth century, with the development of accurate microscopes and of laboratory research, that these processes began to be scientifically elucidated. Several observers indicated the likelihood of microorganisms as causal agents of disease, and even detected their paths of transmission, such as the faecal–oral route for typhoid and cholera, but it was Louis Pasteur who, in the early 1860s, first gave a coherent account of the process of infection in what is popularly known as the germ theory of disease. In 1876, Robert Koch identified the causal organism of anthrax, and within a few years had also identified the agents of tuberculosis and cholera. By 1900, the specific agents of numerous diseases had been identified, and the diverse routes of transmission — of infection and contagion — were beginning to be mapped out.

Infectious diseases are often ‘crowd diseases’, which depend for the most part on reservoirs of susceptible people to maintain themselves. Person to person infections, for example, are thought to have become more apparent between 3000 and 500 bc, when urban centres grew large enough to support them. These diseases soon established an endemic character in such centres, meaning that the diseases or infectious agents were constantly present in that area. City populations, exposed early in life, acquired high levels of immunity to them, compared with rural populations. Rapid and unregulated urban growth brought a great escalation in the incidence of and mortality from many of these diseases in Western Europe and North America during the nineteenth century, and several, including tuberculosis, typhoid, measles, and whooping cough, were responsible for much human misery and many thousands of deaths. Recurrent gastro-intestinal infections, in particular, helped to undermine the health, and natural resistance to infection, of babies and young children, and indeed of adults also. By 1830, annual death rates of over 30 per 1000 living persons were commonplace in Western cities, while infant mortality rates rivalled those of under-developed nations today.

Prevention

Beginning in the 1830s, public health movements began to develop in many Western states in response to this crisis of mortality. For example, in Britain — one of the first nations to begin to adopt public health measures — early reformers such as Edwin Chadwick stressed the enormous economic costs of such a wastage of life. At this period, notions of contagion marched in parallel with a belief that gases generated by rotting organic matter were productive of epidemics, and early attempts at preventing premature deaths focused on environmental improvement. Slowly and painfully, through the following decades, filtered and piped water systems, mains drainage, systematic scavenging, and slum clearance brought about cleaner, healthier urban environments, and disrupted the transmission routes of a number of important infections, notably of water-borne typhoid and cholera and of louse-borne typhus.

The development of specific methods of prevention came late in the history of the infectious diseases. Smallpox, one of the most ancient and most hideous diseases, was the first to be tackled in this way. At some point, the Chinese had discovered that by introducing matter taken from smallpox vesicles into a scratch on the normal healthy body, controlled, immunizing infections could be established. This method, the inoculation of material containing the living organism, itself was not foolproof, since it was not possible to ensure a mild rather than a virulent infection, which might prove fatal. Nonetheless, knowledge of the technique spread along trade routes to Turkey, and thence to Europe in the early eighteenth century. In 1796, a Gloucestershire medical practitioner, Edward Jenner, picked up on local lore which suggested that infections with cow-pox would protect against smallpox, and demonstrated that this was indeed the case. This practice, vaccination (from vaccinus: pertaining to a cow) was later refined, and, encouraged by many European governments, the introduction of the modified or related organism displaced inoculation as the principal preventive against smallpox. At this stage, however, the processes and principles which made vaccination effective were still not understood.

Smallpox vaccination represented an ideal for disease eradication which provided an important model for future medical research. Louis Pasteur, for example, set out in his later career to investigate the principles of immunology with a view to understanding how vaccination worked. Pasteur's breakthrough with the principle of attenuating viruses — reducing their virulence — came in 1876. This meant that the body's immunity to subsequent infection by a virulent organism could be actively provoked in response to a non-threatening form of the same strain; Pasteur proceeded to develop immunizations against various animal diseases, including anthrax and rabies. It was his reluctant application of rabies vaccine to the boy Joseph Meister in 1883 that first alerted the general public to the eventual possibilities of immunology.

As the discipline developed through the work of Pasteur, his colleagues, and his successors, new therapeutic and preventive indications emerged. Early successes came for diphtheria in 1894 with anti-toxin therapy (the use of material produced by the inoculation of animals with toxins produced by bacteria), and for both diphtheria and tetanus with the development of active immunization (the production of protective antibodies by stimulating the body's immune system). In 1896, Almroth Wright succeeded in producing an anti-typhoid vaccine using killed bacteria, thus extending the theoretical options for vaccine development. In the interwar period, successful vaccines were developed against diphtheria and tuberculosis, and, in the years following World War II, they were developed against most of the principal childhood infections — whooping cough, poliomyelitis, German measles, and measles, and eventually against mumps and chicken-pox as well.

Since 1870, there has been an enormous decline in death rates from infectious diseases in developed countries. This decline has been hastened by the availability of immunizations, but in most cases had begun well before such protection was available. Rising living standards — including smaller families, better housing, improved domestic hygiene, a reduced incidence of gastro-intestinal infections, and better nutrition — together with public health measures contributed largely to this reduction. Many childhood diseases remain serious in poor and under-developed countries. Immunization, although a valuable resource with some diseases, is by no means a viable prospect for all infections; despite decades of research, no vaccine has yet been approved for malaria, one of the world's most serious infections.

New infections

New infectious diseases are still emerging, and there is no room for complacency in this regard. The emergence of poliomyelitis as a serious killer and maimer between about 1911 and 1962 was partly attributable to improved hygienic standards in the West, which meant that children were no longer harmlessly exposed to the virus as babies. Lassa fever, exemplar of a whole new generation of sinister tropical fevers, emerged in Nigeria in 1969, while Legionnaires' disease was identified in the US in the 1970s. The rapid global spread of HIV infection since 1980 echoes that of syphilis in Europe in the fifteenth century. Epidemics of the terrifying Ébola virus in Zaire, and of bubonic plague in India in the early 1990s, indicate that both new and old infections retain the potential for major human tragedy. One consequence of global warming could possibly be the reappearance of malaria as an indigenous infection in parts of the world which have been free of it for many decades. Relentless human exploitation of tropical resources, uncontrolled human reproduction, increased travel, and unregulated technological development all create the potential for unleashing fresh manifestations of new and old infections by disturbing global environmental equilibrium.

Anne Hardy

Bibliography

Garrett, L. (1996). The coming plague: newly emerging diseases in a world out of balance. Penguin Books, London.
McNeill, W. H. (1979). Plagues and peoples. Penguin Books, Harmondsworth.

See also antibiotics; epidemic; fever; immune system; immunization; microorganisms; sexually transmitted diseases.