Human beings are restless, nosey and aggressive. These characteristics have made us one of the most invasive species our planet has ever encountered and allowed us to colonise nearly every terrestrial environment. During this progress, humans have made many acquisitions, several of them unwanted. Our constant movement between vast, populous cities and novel environments makes us easy prey for opportunistic pathogens that replicate fast, and transmit by sneezes and dirty hands before sickness even begins to show. These pathogens can spread around the world in hours by aeroplane to infect the unsuspecting on another continent. That's how "swine flu" spread in 2009; it was already unstoppable by the time we noticed it. We were lucky it wasn't particularly virulent.
David Quammen's Spillover is a collection of stories about the sources of diseases called zoonoses. These are the infections that humans acquire from other animals; the most notorious recent example is HIV/Aids, but they encompass a huge range of viruses, bacteria and parasites, including Ebola, some malarias and Q fever. It seems that zoonoses are increasing, at least reports of them are. In 2012 alone, hantavirus (a Sars-like virus), E coli O157 and legionnaires' disease have hit the headlines – all originated in other animals.
Quammen tells us we don't have far to look for blame when weird infectious diseases spring up: they usually result from humans moving into wild ecosystems or from forcing encounters with other species. Bats are at the centre of many of his stories; they seem to be a reservoir for a number of the nastier viruses – Marburg, probably Ebola, Nipah and Hendra viruses – as well as a source of bacterial and fungal infections. Bats are excellent crucibles for pathogens because they often live in huge numbers at high densities, roost in tight proximity and, in part because of habitat loss, congregate around human settlements. Bats could have provided cavemen and women with measles and mumps too. Another particularly acquisitive period in human evolution was probably livestock domestication about 10,000 years ago. This seems to have been the major trigger not just for human civilisation, but also for the accumulation of the familiar repertoire of communicable human diseases.
If they are not made locally extinct by our activities, other animal species either hang on at the fringes of human settlements, or take to suburban living. In the US, homes in woodland idylls are aspirational dwellings, but the human invasion has made Lyme disease a widespread problem. Having lost their own space, the wild reservoir hosts of this bacterial infection – mice and deer, with their tick vectors – are forced to colonise the newly made backyards and come into contact with humans. Likewise, intimate living with rats in chaotic medieval cities allowed plague to spread worldwide, and as rat populations thrive in modern cities too, it wouldn't take much of a drop in living standards to give plague a new foothold.
Smaller, highly mobile creatures also bring unwanted pathogens into human spaces. Mosquitoes can make the transition from forest to city living strikingly easily. They ride in planes and have taken with them terrible afflictions, such as West Nile virus to the US, and have spread dengue virus in Asia and South America. Mosquitoes breed fast, are genetically flexible, and can adapt rapidly to change their feeding and flying behaviour, tolerate climate shifts and develop insecticide resistance. Some are adept at breeding in almost any drop of uncovered water, for instance in an old tin can, a discarded tyre, or a hollow plant stem. This way, Asian tiger mosquitoes have spread to Europe, where they now breed and are known to have transmitted chikungunya virus to Italy.
Quammen has not considered – perhaps because we have only recently had the technology to witness it – the extraordinary amount of gene swapping among pathogens. Some of the most mobile of these genes supply components for antibiotic resistance and virulence, and may help a pathogen to change its host. This genetic movement, in combination with the profligate use of antibiotics, has now rendered a large armamentarium of once life-saving drugs useless.
Zoonosis is a topic that, like the pathogens themselves, is hard to keep under control, and Quammen's treatment is rather uneven. Perhaps unsurprisingly, he veers from inflamed alarmism to cold, sober mathematics. I started off feeling irritated by asides such as "Here's what you have to come to understand …" But he is terrific when on a quest to uncover the circumstances that led to a particular spillover event. The stories of the victims and the scientists are told in astonishing detail. He has named nearly all the local heroes of epidemiology and made several forays into west Africa in the process. He's at his best towards the end of the book in the section devoted to HIV, although soon afterwards he becomes so impassioned he inserts long flights of fictional fancy about the "Voyager" who brings HIV into the wider world. The book needed fiercer editing.
Like bats, humans now live together in huge numbers, tightly packed and forever flitting from place to place. We are prime candidates for, as Quammen puts it, the Next Big One. What will it be? Perhaps an RNA virus, like the one portrayed in Steven Soderbergh's film Contagion, but probably one more banal – influenza. Again though, we can teeter from the routine consideration of seasonal influenza (which still kills thousands of people each year) to scaremongering about bird flu (which has so far killed only a few hundred – though more than half of those infected died).
The problem is that, for all its apparent simplicity, the flu virus hides its secrets well. It is constantly changing to acquire and shed mutations, at the same time gaining and losing virulence and shifting host adaptability. Making the annual vaccine for seasonal flu is an industrial-scale challenge of diplomacy and logistics, masterminded by the World Health Organisation. Influenza drugs are poor and resistance to them builds up quickly during an epidemic. So although we know a lot about flu, there is a lot more we don't know. We can't predict with much accuracy when the next spillover from birds will happen, whether or not it will spawn a devastating pandemic killing tens of millions, or where, how and what subtype it will be. We only know we still don't have the surveillance systems in place to stop it in time.
