In the 5th and 14th century, Plague epidemics spread internationally and killed approximately 15% of the global population over the course of a few decades. Systematic vaccination campaigns have allowed us to eradicate two diseases that had affected humanity for centuries, Smallpox in humans and Rinderpest in animals, and two more diseases – Guinea Worm and Polio – are close to being eradicated. Progress in medical treatment and public health systems has significantly reduced the prevalence and impact of others, such as Malaria, Typhus and Cholera. However, there remains a serious risk that the emergence of a new infectious disease in humans could cause a major outbreak, with particularly high mortality and rapid spread in our densely populated, urbanized and highly interconnected world.
Catastrophic pandemics – diseases with high lethality that spread globally – are extremely disruptive, but very rare. Outbreaks of lethal diseases that remain locally contained or pandemics with less acute effects on human health are however more common, and can have significant disruptive effects.
Outbreaks occur when a micro-organism – virus, bacteria, parasite, etc. – is able to spread across the population. At times and under certain conditions, such as failure of water or sanitation systems, an outbreak is caused by a micro-organism known to be circulating at low levels in human populations. At others, an outbreak is caused by a micro-organism that has crossed the animal/human species barrier to infect humans, and spreads to new and more densely populated areas. If mutation occurs, virulence can increase or decrease. Mutation can also cause a micro-organism to transmit more easily from human to human.
In February 2003, an elderly woman infected by the SARS virus travelled from Hong Kong to Toronto. SARS is a highly infectious and often fatal pulmonary disease that emerged in the Pearl River Delta, in China. The infected woman died soon afterwards in Toronto, after inadvertently infecting over forty people, resulting in a localized outbreak. One of those persons infected in Canada went on a plane to the Philippines, where another outbreak occurred. Meanwhile, from Hong Kong, the virus had also spread to Singapore, where it likewise caused an outbreak. The outbreaks that occurred around the world were eventually contained, after infecting over 8,000 people, of whom 774 died, through concerted public health action coordinated by the WHO. Severe social and economic disruption occurred, and a similar scenario with only minor variations – a few more international contacts, a slightly longer incubation period for the virus, or a few more days of delay in deploying strict containment measures, could have a similar or even greater outcome.
Three main factors determine the potential danger of an outbreak:
Ebola is a highly lethal infection with a short incubation period but a relatively low infection rate, which explains why most Ebola outbreaks to date have been localized. New developments in synthetic biology, however, raise concern among certain scientists that an engineered micro-organism both highly virulent and with a high infection rate could be released in the population – whether by malice or accident – and cause an unprecedented outbreak, possibly leading to the international spread of a highly lethal infectious disease.
Antibiotics have saved millions of lives and dramatically increased lifespans since they were introduced in the 1940s, allowing us to contain most bacterial infections and diseases. However, more recently, as a result of random mutations, improper use of antibiotics among humans and animals, and the buildup effects of evolution, some strains of bacteria have become resistant to traditional antibiotics. These ‘superbugs’ require alternative medications with more damaging side effects or, in the worst cases, can no longer be treated effectively. Antibiotic-resistant bacteria currently kill an estimated 700,000 people each year worldwide. That number is predicted to reach 10 million by 2050 if efforts are not made to curtail resistance or develop new antibiotics.
Head and Senior Fellow, Centre on Global Health Security, Chatham House, Professor of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine