Outbreaks are a much larger menace today than they were just three decades ago. They are larger in two ways.

First, changes in the way humanity inhabits the planet have led to the emergence of new diseases in unprecedented numbers. In the thirty years from 1973 to 2003, when SARS appeared, 39 pathogenic agents capable of causing human disease were newly identified.

The names of some are notoriously well-known: Ebola, HIV/AIDS, and the organisms responsible for toxic shock syndrome and legionnaire’s disease. Others include new forms of epidemic cholera and meningitis, Hanta virus, Hendra virus, Nipah virus, and H5N1 avian influenza.

This is an ominous trend. It is historically unprecedented, and it is certain to continue.

Second, the unique conditions of the 21st century have amplified the invasive and disruptive power of outbreaks. We are highly mobile. Airlines now carry almost 2 billion passengers a year. SARS taught us how quickly a new disease can spread along the routes of international air travel. Financial markets are closely intertwined. Businesses use global sourcing and just-in-time production. These trends mean that the disruption caused by an outbreak in one part of the world can quickly ricochet throughout the global financial and business systems. Finally, our electronic interconnectedness spreads panic just as far and just as fast.

This has made all nations vulnerable – not just to invasion of their territories by pathogens, but also to the economic and social shocks of outbreaks elsewhere. Some experts have gone so far as to state that there is no such thing as a “localized” outbreak anymore. If the disease is lethal, frightening, or spreading in an explosive way, there will always be international repercussions.

The best defence against emerging and epidemic-prone diseases is not passive barriers at borders, airports and seaports. It is proactive risk management that seeks to detect an outbreak early and stop it at source – before it has a chance to become an international threat. We as a public health experts are now in a good position to act in this pre-emptive way.

The opportunities are multiple. The pressures of population growth push people into previously uninhabited areas, disrupting the delicate equilibrium between microbes and their natural reservoirs. This creates opportunities for new diseases to emerge.

Population growth also puts people in close proximity to domestic animals, creating evolutionary pressures and opportunities for pathogens to jump the species barrier. Of the emerging pathogens capable of infecting humans, around 75% originated as diseases of animals.

Environmental degradation and changing weather patterns allow known diseases to flare up in unexpected places, at unexpected times, and with unprecedented numbers of cases.

Intensive food production, including the use of antibiotics in animals, creates additional pressures on the microbial world, leading to mutations and adaptations, including drug resistance.

And we must not forget: drug-resistant strains of viruses and bacteria also travel well internationally.

International health security focuses attention on these complex and interrelated threats to our collective security.

They reinforce our need for shared responsibility and collective action in the face of universal vulnerability, in sectors well beyond health.