Deadly encounters: an infectious world

Throughout Australian history epidemics of infectious diseases have come and gone. Measles, influenza, diphtheria, smallpox, plague, scarlet fever, dengue and polio have all played an important part in our history and reaped a substantial toll of human lives. Most of these disease outbreaks captured public attention and led to great outpourings of fear, hysteria and panic. Many also became the prime cause of public health reform.

More recently Australia and the world have struggled with HIV/AIDS, SARS, Avian and Swine Flu. Looking back, infectious disease has been responsible for the deadliest episodes in Australian history including the influenza pandemic of 1918 which caused more than 15,000 deaths and hundreds of thousands of cases, the smallpox epidemic of 1913-14 which affected New South Wales and probably produced well over 3,000 cases, the measles epidemic of 1938 which resulted in more than 4,000 cases or the polio epidemics between 1904 and 1961 which were responsible for at least 30,000 cases and immeasurable suffering.

More recently, HIV/AIDS and various forms of flu have continued to be a major problem. There is little doubt that infectious diseases have been emerging for centuries, perhaps millennia. Although we often like to think that our 21^st century world is an infectious disease-free world such diseases constitute a substantial fraction of all those that regularly assail us, whether they be gastro-intestinal infections, respiratory infections, sexually-transmitted infections, childhood infections or epidemic threats from abroad.

Importantly, many that now threaten us are zoonoses or animal diseases, often permanently maintained in the microclimate of the burrows of small ground living rodents or bats.

The ebola outbreak in West Africa is the latest example of such a disease. This outbreak, which seems to be currently totally out of control, has already affected more than 60 localities in three countries. The fear of it spreading further afield remains a real issue. It seems that we live in a world where infectious disease is inevitable and unpredictable, and despite our measures of control, the rapidity and ease of world travel, the difficulty of indentifying potential cases of disease before symptoms appear, growing urbanization, climate change, increasing global interdependence and the vagaries of human behavior have greatly added to the task of containing disease outbreaks.

Today two major forms of infectious disease continue to confront us. Newly emerging infections and re-emerging infections. Re-emerging infections are those which have never disappeared but reappear either in their original form in new locations or in a slightly modified form. What we call "new" infections may in fact not be "new" at all but simply infections that have existed among their animal hosts for centuries before making the jump to us often via an intermediary host like pigs or chickens.

But what does the future hold? Will we ever eliminate or indeed be able to control infectious disease?

Smallpox remains our only significant victory in the battle against infectious disease. Other infections we have managed to largely control, although given the nature of human behavior and the ability of infectious diseases to develop resistance to our antibacterial and anti-viral drugs, that control is at best somewhat tenuous.

Take for example the case of pertussis or whooping cough in Australia. Even though the current vaccine provides a shorter and somewhat less effective immunity than we once thought, vaccination of children, should allow us to control the disease. But Australia has had more than 113,000 cases of pertussis between 2010 and 2014 when there should have been very few, if any. Human behavior rules and many people simply refused to have their children vaccinated.

And when it comes to 'control' during times of epidemic crisis we inevitably reach back into the past and rediscover quarantine, border closures, isolation of cases and contacts, cleansing and scavenging, restrictions on travel, aerial sprays and masks. Our arsenal of antibiotic and anti-viral drugs, vaccines and pesticides also always seems to be one step behind the ability of our target to adapt and mutate into a new more deadly form. And then there is the continuing intrusion on, and modification of, isolated rural ecosystems particularly in Africa and Asia, usually carried out by governments and national and international corporations in the so-called interest of development and modernization. Such activities often disturb areas where infections like plague, influenza and ebola have existed in natural animal reservoirs for centuries.

The outcome may very well be new lethal waves of viral and bacterial disease.

We also need to fully appreciate that we live in a world that is continually in motion and that the global movement of people and products can have devastating effects. In many ways the unprecedented volume, speed and range of human movement is one of the main characteristics of globalization. From refugees fleeing war, civil unrest or natural disasters, to international tourists or business travelers, more people are on the move than ever before. Not only that but they are moving faster and further than ever before, as well as regularly visiting more remote and exotic locations.

As well, rampant urbanization in Africa, Asia and Latin America, driven largely by large scale rural-urban migration, has produced high population densities, adequate housing, poor waste disposal and poor or totally absent sanitation.

Such conditions have encouraged the spread of a wide range of infectious diseases as well as creating an ideal environment for disease vectors like mosquitos to breed.

We should also continue to marvel at the ability of the microbial world to survive and replicate almost at will, as well as to mutate and change according to changes in their environment and that of their host. Above all we need to fully realise that the world we inhabit today is really a world dominated by microbes and that we are merely short-term tenants.