Terrible new forms of infectious disease make headlines, but not at the start. Every pandemic begins small. Early indicators can be subtle and ambiguous. When the Next Big One arrives, spreading across oceans and continents like the sweep of nightfall, causing illness and fear, killing thousands or maybe millions of people, it will be signaled first by quiet, puzzling reports from faraway places--reports to which disease scientists and public health officials, but few of the rest of us, pay close attention. Such reports have been coming in recent months from two countries, China and Saudi Arabia.

You may have seen the news about H7N9, a new strain of avian flu claiming victims in Shanghai and other Chinese locales. Influenzas always draw notice, and always deserve it, because of their great potential to catch hold, spread fast, circle the world and kill lots of people. But even if you’ve been tracking that bird-flu story, you may not have noticed the little items about a “novel coronavirus” on the Arabian Peninsula.

This came into view last September, when the Saudi Ministry of Health announced that such a virus--new to science and medicine--had been detected in three patients, two of whom had already died. By the end of the year, a total of nine cases had been confirmed, with five fatalities. As of Thursday, there have been 18 deaths, 33 cases total, including one patient now hospitalized in France after a trip to the United Arab Emirates. Those numbers are tiny by the standards of global pandemics, but here’s one that’s huge: the case fatality rate is 55 percent. The thing seems to be almost as lethal as Ebola.

Coronaviruses are a genus of bugs that cause respiratory and gastrointestinal infections, sometimes mild and sometimes fierce, in humans, other mammals and birds. They became infamous by association in 2003 because the agent for severe acute respiratory syndrome, or SARS, is a coronavirus. That one emerged suddenly in southern China, passed from person to person and from Guangzhou to Hong Kong, then went swiftly onward by airplane to Toronto, Singapore and elsewhere. Eventually it sickened about 8,000 people, of whom nearly 10 percent died. If not for fast scientific work to identify the virus and rigorous public health measures to contain it, the total case count and death toll could have been much higher.

One authority at the Centers for Disease Control and Prevention, an expert on nasty viruses, told me that the SARS outbreak was the scariest such episode he’d ever seen. That cautionary experience is one reason this novel coronavirus in the Middle East has attracted such concern.

Another reason is that coronaviruses as a group are very changeable, very protean, because of their high rates of mutation and their proclivity for recombination: when the viruses replicate, their genetic material is continually being inaccurately copied--and when two virus strains infect a single host cell, it is often intermixed. Such rich genetic variation gives them what one expert has called an “intrinsic evolvability,” a capacity to adapt quickly to new circumstances within new hosts.

But hold on. I said that the SARS virus “emerged” in southern China, and that raises the question: emerged from where? Every new disease outbreak starts as a mystery, and among the first things to be solved is the question of source.

In most cases, the answer is wildlife. Sixty percent of our infectious diseases fall within this category, caused by viruses or other microbes known as zoonoses. A zoonosis is an animal infection transmissible to humans. Another bit of special lingo: reservoir host. That’s the animal species in which the zoonotic bug resides endemically, inconspicuously, over time. Some unsuspecting person comes in contact with an infected monkey, ape, rodent or wild goose--or maybe just with a domestic duck that has fed around the same pond as the wild goose--and a virus achieves transcendence, passing from one species of host into another. The disease experts call that event a spillover.

Researchers have established that the SARS virus emerged from a bat. The virus may have passed through an intermediate species--another animal, perhaps infected by cage-to-cage contact in one of the crowded live-animal markets of the region--before getting into a person. And while SARS hasn’t recurred, we can assume that the virus still abides in southern China within its reservoir hosts: one or more kinds of bat.

Bats, though wondrous and necessary animals, do seem to be disproportionately implicated as reservoir hosts of new zoonotic viruses: Marburg, Hendra, Nipah, Menangle and others. Bats gather in huge, sociable aggregations and have long life spans, circumstances that may be especially hospitable to viruses. And they fly. Traveling nightly to feed, shifting occasionally from one communal roost to another, they carry their infections widely and spread them to one another.

As for the novel coronavirus in Saudi Arabia, its reservoir host is still undiscovered. But you can be confident that scientific sleuths are on the case and that they will look closely at Arabian bats, including those that visit the productive date-palm groves at the oases of Al Ahsa, near the Persian Gulf.

What can we do? The first obligation is informed awareness. Early reports arrive from afar, seeming exotic and peripheral, but don’t be fooled. One emergent virus, sooner or later, will be the Next Big One. It may show up first in China, in Congo or Bangladesh, or maybe on the Arabian Peninsula; but it will globalize. Most people on earth nowadays live within 24 hours’ travel time of Saudi Arabia. And in October, when millions of people journey to Mecca for the hajj, the Muslim pilgrimage, the lines of connections among humans everywhere will be that much shorter.

We can’t detach ourselves from emerging pathogens either by distance or lack of interest. The planet is too small. We’re like the light heavyweight boxer Billy Conn, stepping into the ring with Joe Louis in 1946: we can run, but we can’t hide.