BOZEMAN, Mont.

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.