A new global health crisis: emerging infectious diseases

Until quite recently, the global war against infectious diseases appeared to be on the road to victory. However, the advent of new infectious diseases and the re-emergence of old ones in areas of the world avowedly free of such diseases, have precipitated a new health crisis which threatens to overwhelm the gains so far made.

UNTIL a relatively few years ago, there was a sense of opti- mism that the long struggle for control over infectious diseases was almost over. Smallpox was eradicated. Poliomyelitis, guinea-worm disease, leprosy, Chagas disease and neonatal tetanus were targeted for eradication or elimination. Some 8 out of 10 of the world's children were immunised against half a dozen killer diseases. Antimicrobial drugs were effectively suppressing countless infections.

But cautious optimism has turned into a fatal complacency that is costing millions of lives every year. The struggle for control, far from being over, has reached a critical stage. Infectious diseases remain the world's leading cause of death, accounting for at least 17 million (about 33%) of the 52 million people who die each year. Apart from those 17 million - about 9 million of whom are young children - up to half the world's population of 5.72 billion people are at risk of many endemic diseases.

Diseases that used to be restricted geographically, such as cholera, are now striking in regions once thought safe. While some diseases have been almost completely subdued, others such as malaria and tuberculosis that have always been among our greatest enemies are fighting back with renewed ferocity. The role of infectious agents in the development of many types of cancer is becoming more evident.

New diseases, new threats

New and emerging diseases, combined with the rapid spread of pathogens resistant to antibiotics and of disease-carrying insects resistant to insecticides, are daunting challenges to human health. The new diseases range from AIDS to little-known but equally lethal viral infections. In many cases, their source is unknown, as is the reason for their emergence. More often than not, no specific treatment is yet available for them.

Antibiotic resistance in hospitals worldwide threatens to leave medical and public health workers virtually helpless in the prevention or treatment of many infections. Many of the most powerful antibiotics have been rendered impotent. Disastrously, this is happening at a time when too few new drugs are being developed to replace those that have lost their effectiveness. In the contest for supremacy, the microbes are sprinting ahead. The gap between their ability to mutate into drug-resistant strains and man's ability to counter them is widening fast.

These are tragic developments, given the achievements that have been made in global disease control. The discovery of antibiotics, the development of vaccines and, more importantly, the introduction of environmental sanitation measures coupled with better understanding of infectious disease epidemiology, have been invaluable weapons in the fight for human health.

The price of failure

The re-emergence of infectious diseases is a warning that progress achieved so far towards global security in health and prosperity may be wasted unless effective development policies are formulated, and commitments are made to implement them nationally and internationally.

Infectious diseases range from those occurring in tropical areas (such as malaria and dengue haemorrhagic fever, which are most common in developing countries) to diseases found worldwide (such as hepatitis and sexually transmitted diseases, including HIV/AIDS) and food-borne illnesses that affect large numbers of people in both the richer and the poorer nations.

The struggle for control

A few examples illustrate the impact of infectious diseases on human health and development:

* Malaria - the worst of the insect-borne diseases - still strikes up to 500 million people a year, killing at least 2 million.

* Acute lower respiratory infections kill almost 4 million children every year. Tuberculosis, similarly spread from person to person, kills 3 million people a year and one-third of the global population carries the bacilli that cause it.

* Diarrhoeal diseases, spread chiefly by contaminated water or food, kill nearly 3 million young children every year. Cholera epidemics are occurring in countries ranging from South-East Asia to the Middle East, and as far apart as western Africa and South America.

* HIV, the virus that causes AIDS, is predominantly transmitted sexually, and has already infected up to 24 million adults, of whom at least 4 million have died. More than 330 million new cases of other sexually transmitted diseases occurred in 1995.

* Viral hepatitis is another major problem worldwide. The term is used to describe a group of several distinct infections which are similar in many ways, but which nevertheless differ in some of their characteristics, and in their prevention and control. At least 350 million people are chronic carriers of the Hepatitis B virus, and another 100 million are chronic carriers of the Hepatitis C virus. At least a quarter of them will die of related liver disease.

* Some of the 10 million new cases of cancer diagnosed in 1995 were caused by viruses (Hepatitis B and Hepatitis C among them), bacteria and parasites. WHO estimates that 15% of all new cancer cases could be avoided by preventing the infectious diseases associated with them.

Breaking the chains

In all types of infectious disease, prevention and control depend on breaking the chains of transmission. A handful of diseases are within range of elimination or eradication in the next few years and others are under control, thanks largely to effective public health measures (particularly global immunisation programmes) and other interventions. Poliomyelitis and guinea-worm disease, for example, could be eradicated by the end of the century. Other diseases, such leprosy, could be eliminated as public health dangers by reducing their prevalence to a very low level.

Attaining freedom from infectious disease is one of humanity's foremost preoccupations. But breaking the chains that shackle people to these diseases is an immensely difficult task. It has been fully achieved only once, with the last reported case of smallpox in 1977. The fact that such success has not been repeated is due not primarily to lack of knowledge or tools, but more to logistics problems and a series of events and developments, some natural and others man-made, that have occurred in recent years. Some are poverty-related, while others are the consequences of economic prosperity.

Obstacles to success

Poverty is on the increase, so that hundreds of millions of people are bound by their living conditions to the daily hazard of infectious disease. More than one-fifth of the world's population lives in extreme poverty. Almost a third of all children are undernourished. Half the people in the world lack regular access to the most needed essential drugs.

Continuing global population growth, combined with rapid urbanisation, means that many millions of city dwellers are forced to live in overcrowded and unhygienic conditions, where lack of clean water and adequate sanitation provides breeding grounds for infectious disease. High-density populations raise the risk of respiratory disease and those transmitted through contact with pathogens in food and water.

In addition, more than 90% of expected population growth in the coming decades will be in the developing regions of Africa, Asia and Latin America - the regions of richest biological diversity. Human encroachment on tropical forests has brought populations with little or no disease resistance into close proximity with insects that carry malaria and yellow fever and other, sometimes unknown, infectious diseases.

Because of the economic and social crises that still affect many countries, health systems which should offer protection against disease have, in extreme cases, either collapsed or not even been built. The immediate result is a resurgence of diseases that were once under control or should be controllable, given adequate resources. Disabled by these diseases, some societies are unable to get themselves back on their feet and cannot afford the health services that they need.

Migration and the mass movement of millions of refugees or displaced persons from one country to another - as the result of wars, civil turmoil or natural disasters - also provide fertile breeding grounds for infectious diseases and keep them on the move.

Increases in international air travel and the growing traffic in trade and tourism mean that disease-producing organisms, the deadly as well as the commonplace, can be transported rapidly from one continent to another. There are now about 5,000 airports with scheduled worldwide services. Air travel has increased by almost 7% a year in the last 20 years and is predicted to increase by over 5% a year during the next 20 years.

As a result of changes in human sexual behaviour, more people are exposed to sexually transmitted diseases. Worldwide, almost a million people contract such diseases every day, mostly young adults.

Other changes in global food trade create new opportunities for infections to flourish. They include the shipment of livestock; food production, storage and marketing; and altered eating habits.

Social changes place certain age groups at higher risk from infectious diseases - for example the clustering of young children in day-care centres, and the growing numbers of the elderly in nursing homes.

Expanding areas of human habitation put additional millions of people at risk from pathogens previously rare or unknown causes of human disease. The effects of climatic change may give some diseases the opportunity to spread to new geographical areas. Antimicrobials have become ubiquitous. Microbes, meanwhile, continue to evolve and adapt to their environment, as they have always done, adding anti-microbial resistance to their evolutionary pathways.

For all the reasons given above, controlling infectious diseases remains global challenge. In addition, the social and economic costs of infectious diseases are far from negligible for individuals, families and communities.

The call for action is directed not just at those working in the field of health, since the reasons for the present crisis are multiple. They are political, social, environmental and to a significant extent self-inflicted - the by-products of the modern world.

The above is an excerpt from the World Health Report 1996 (pp 1-4) prepared by the World Health Organisation, Geneva.