its killing so slowly. Most other new viruses work fast.
I’ve been using the words “emergence” and “emerging” as though they are everyday language, and maybe they are. Among the experts, they’re certainly common parlance. There’s even a journal dedicated to the subject, Emerging Infectious Diseases, published monthly by the CDC. But a precise definition of “emergence” might be useful here. Several have been offered in the scientific literature. The one I prefer simply says that an emerging disease is “an infectious disease whose incidence is increasing following its first introduction into a new host population.” The key words, of course, are “infectious,” “increasing,” and “new host.” A re-emerging disease is one “whose incidence is increasing in an existing host population as a result of long-term changes in its underlying epidemiology.” Tuberculosis is re-emerging as a severe problem, especially in Africa, as the TB bacterium exploits a new opportunity: infecting AIDS patients whose immune systems are disabled. Yellow fever re-emerges among humans wherever Aedes aegypti mosquitoes are allowed to resume carrying the virus between infected monkeys and uninfected people. Dengue, also dependent on mosquito bites for transmission and native monkeys as reservoirs, re-emerged in Southeast Asia after World War II due at least partly to increased urbanization, wider travel, lax wastewater management, inefficient mosquito control, and other factors.
Emergence and spillover are distinct concepts but interconnected. “Spillover” is the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another. It’s a focused event. Hendra virus spilled over into Drama Series (from bats) and then into Vic Rail (from horses) in September 1994. Emergence is a process, a trend. AIDS emerged during the late twentieth century. (Or was it the early twentieth century? I’ll return to that question.) Spillover leads to emergence when an alien bug, having infected some members of a new host species, thrives in that species and spreads among it. In this sense, the strict sense, Hendra hasn’t emerged into the human population, not yet, not quite. It is merely a candidate.
Not all emerging diseases are zoonotic, but most are. From where else might a pathogen emerge, if not from another organism? Well, granted, some novel pathogens do seem to emerge from the environment itself, without need for shelter in a reservoir host. Case in point: The bacterium now called Legionella pneumophila emerged from the cooling tower of an air-conditioning system at a hotel in Philadelphia, in 1976, to create the first-known outbreak of Legionnaires’ disease and kill thirty-four people. But that scenario is far less typical than the zoonotic one. Microbes that infect living creatures of one kind are the most likely candidates to infect living creatures of another kind. This has been borne out statistically by several review studies in recent years. One of them, published by two scientists at the University of Edinburgh in 2005, looked at 1,407 recognized species of human pathogen and found that zoonotic bugs account for 58 percent. Of the full total, 1,407, just 177 can be considered emerging or re-emerging. Three-fourths of those emergent pathogens are zoonotic. In plain words: Show me a strange new disease and, most likely, I can show you a zoonosis.
A parallel survey, from a team led by Kate E. Jones of the Zoological Society of London, appeared in the journal Nature in 2008. This group considered more than three hundred “events” of emerging infectious disease (EIDs, in their shorthand) that occurred between 1940 and 2004. They wondered about changing trends as well as discernible patterns. Although their list of events was independent of the Edinburgh researchers’ list of pathogens, Jones and her colleagues found almost the same portion (60.3 percent) to be zoonotic. “Furthermore, 71.8% of these zoonotic EID events were caused by pathogens with a wildlife origin,” as distinct from domestic animals. They cited Nipah in Malaysia and SARS in southern China. Further still, the increment of disease events associated with wildlife, as opposed to livestock, seems to be increasing over time. “Zoonoses from wildlife represent the most significant, growing threat to global health of all EIDs,” these authors concluded. “Our findings highlight the critical need for health monitoring and identification of new, potentially zoonotic pathogens in wildlife populations, as a forecast measure for EIDs.” That sounds reasonable: Let’s keep an eye on wild creatures. As we besiege them, as we corner them, as we exterminate them and eat them, we’re getting their diseases. It