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.