PROLOGUE "I Ask This Global Community to Pause" In Lynn Henry''s Words Sometimes you don''t see the warning until it''s too late. Sometimes you hear the warning but fail to heed its message. And sometimes you see, hear, and understand--but the symphonic roar of the world drowns out your solo note of alarm. A single tragedy unites us all in the end, though: our many small, casual, disbelieving, distracted, unsure, risk-calculated, understandable, self-serving, self-sacrificing, protective, recalcitrant, completely unaware, and very particular failures to see, hear, and communicate reveal their true meaning only on the other side of the impassable divide between "then" and "now." This story begins with the end of "then." On New Year''s Eve, as 2019 was silently, invisibly mutating into 2020, my sister Bonnie and I were, unusually, together. Normally I would have flown to Prince Edward Island from Toronto, where I live, to spend the holidays with my parents there. And Bonnie habitually spent the same period in her beloved home of British Columbia, where two years earlier she had been appointed the province''s "top doctor," or medical officer of health--the first woman to hold that position.

The last time we''d spent New Year''s in each other''s company had been more than twenty years before, when Bonnie was in San Diego, finishing a degree in public health and working as a family doctor at an inner-city medical clinic. I had joined her from Canada that long-ago December, and I remember visiting the clinic one afternoon and Bonnie calmly pointing out the pockmarks of bullet holes in a waiting-room wall. She explained that the building housing the clinic happened to sit at an intersection between the streets of rival gangs. Occasionally there would be drive-by shootings, and staff and patients would duck for cover, make sure there were no casualties outside or in, and carry on. As 2019 shape-shifted into 2020, however, Bonnie and I were seemingly as far away from that earlier time and space as you could get, sitting quietly on a balcony under a slim crescent moon, overlooking a cliff that sloped down to the Caribbean Sea. Our uncle had for decades owned a suite in the beautiful old hotel where we were staying, and out of the blue he had offered us the space for ten days during this quietest time of year; he, like most of the regulars, would arrive for a much longer stretch in late January, escaping the wintry Prairies. Bonnie had been exhausted in drizzly Victoria, I was bone-weary in grey Toronto, and we''d both perked up equally at this surprise invitation. Now, six days in, we sat outside in the soft dark as Bonnie told me tales of her time on this very island years earlier--while still in medical school, she and a windsurf-loving colleague had spent a semester learning and practising emergency medicine at a hospital in the nearby capital city, travelling the coast in search of waves on their days off.

Just before midnight, we sipped a celebratory whisky in companionable silence. I listened to the sea breaking against the cliff-foot, thinking how the sound was so very strange in this moment, yet as familiar to me as breath, just as my sister''s presence on this particular day was so strange and familiar all at once, our lives like two strands in a helix mysteriously crossing at key points; and I was reminded of our parents on another fragile island, no doubt frozen and blustery just then, in the North Atlantic. I thought, too, of the prime minister of this place whose waves were right now a calm, regular rhythm in my ears, the extraordinary Mia Mottley, and her unsettling, searing words a year before to the UN about the plight of small island nations in our time of climate change. "In good conscience, I cannot give the speech that I prepared," she had told the world then, as she arranged to cut her trip short and rush home to deal with devastating storms and severe flooding. "We, as a small state, are used to being treated as if we didn''t exist . [But] what happened in the last twenty-four hours is not a science fiction movie. We must have caring and empathy . It is not about governments anymore," she said.

"It is about people. I ask this global community to pause. Time is running out." "I''m worried about what will happen next year," Bonnie said suddenly into the night sky, as if spying the trail of my thoughts. "The health minister and I haven''t talked directly for a while. That''s my responsibility, too, of course. But we need to align our ideas on how to communicate about the overdose crisis." On cue, a loud alarm started up, its siren interrupting her words.

We both stood and leaned over the balcony, peering at the pool belonging to the ground-floor suite below. Its water rippled in the light breeze, but otherwise pool and suite appeared undisturbed and empty, as they had been all week. Still the siren continued, harsh and insistent. "Well, something''s set it off," Bonnie said. "Maybe an animal tripped it. I''ll call the front desk and ask if security might check it out and hopefully make it stop." As I looked past the glow of underwater lights around the edge of the pool into the darker waving shadows of the tall grasses leading to the edge of the cliff, another, perhaps ridiculous, possibility occurred to me. "I think it might be frogs," I ventured.

"Not an alarm at all." Bonnie shook her head. "No, definitely not. I''m calling the night desk." I admire Bonnie''s characteristic certainty, a quality she has possessed since childhood, because I know it''s informed by a clear-eyed understanding of the facts and a measured consideration of the probabilities. I weighed this against my own way of seeing and mulling multiple competing possibilities at once. Usually, in our relationship, mine was the losing proposition. "Humour me?" I said after a pause, and perhaps it was the smoky whisky or the beckoning warmth of the air or the worry that I would set off stubbornly on my own, but Bonnie sighed and did.

Soon we were scrambling through thorny hedges and high grass. We reached a clearing, and I stooped to find a rock, the siren now on a dreadful, ever-louder repeating loop. With little hope, I flung the rock in the direction of the cliff and heard a dull thunk . Then: magnificent silence. Bonnie and I each held an inhalation for a long moment--and the silence held, too. In the final seconds of the old year, we laughed and laughed, relieved and a little embarrassed as we made our way back, only the sound of the sea exhaling behind us. But somewhere, I know now, there really was an alarm ringing. I just couldn''t hear its unusual frequency, whereas my sister--her senses heightened by a nimbus of PTSD from her experience twenty years earlier fighting SARS; by her current pitch of constant anxiety, honed over the past year''s desperate, and frankly losing, fight to control the overdose crisis raging in her province; and by her never-sleeping internal weather system that was already gathering and synthesizing still-cloudy signs and signals coming out of the World Health Organization--could.

In less than a month, back in Canada, she and her province''s health minister, now in almost daily contact, would hold their first joint press conference about a stealthy new virus spreading breath by breath throughout the world. In Dr. Bonnie Henry''s Words Most of us go through our days blissfully unaware of the constant small cues the world sends out about potential hazards that may invade our lives: heat alerts that could translate into wildfires raging out of control; weather advisories that may morph into hurricanes and storm surges; recall notifications of salmonella-laced spinach whose consumption might lead to hundreds being sickened. For those of us on the frontlines of public health, though, these are the modern tickertapes that flow through our days. Whether they''re alerts on the global ProMED listserv that tell of unusual cases and outbreaks of illness in all corners of the globe or random online articles that mention a doctor in China raising an alarm, we follow these signs carefully--and perhaps, for some of us, with something akin to religious fanaticism. As we watch the world move en masse and people go about their daily business, these signs pop up in our consciousness as warnings. But which ones are signals of something bigger, more ominous? That is the challenge and the work of public health experts across the globe. We watch the signs in our local communities; in our provinces, states, or regions; in our countries; and collectively around the world through the World Health Organization.

We''re a minuscule part of the health system in countries everywhere, and mostly behind the scenes, but we''re also essential, preventing illness and injury, protecting and promoting health in all its forms. And every now and then we emerge from the shadows to play a critical role, leading the response to threats that can range from influenza to Ebola, to food-borne illness, to lead in drinking water, to the effects of climate change and radiation. In these moments, public health teams globally become the front lines. You may not realize it, but when you develop such symptoms as the characteristic cough and fever of influenza and seek care from your local doctor, at that moment you may enter an international network set up to monitor the spread of key infections and detect new pathogens, whether it''s a novel strain of influenza or something completely different. Our surveillance systems include labora.