Twitter execs realized they couldn’t afford to stand by until the science and official guidance were settled. “If we wait for enough data to present itself for a jurisdiction to make a confident decision,” Christie remembers thinking, “we might be too late for our own people.”

On Wednesday, February 26, the executive team ordered every Twitter employee in Japan to begin working at home immediately. Two days later workers in Korea were told to stay home. A directive for the rest of the company’s staff wasn’t far behind. On Friday, the 28th, Christie stepped into a conference room with CEO Jack Dorsey and other executives. “We decided that on Monday we had to strongly encourage everybody to stay home,” she says.

By the end of that week, Lyft, Facebook, Google, Apple, and Salesforce had all followed Twitter’s example, and San Francisco discovered its first two cases of local residents with Covid-19. Collectively, these companies eventually ordered many tens of thousands of people in the Bay Area to work from home. This had both the powerful practical effect of taking people out of circulation and an arguably greater cultural effect.

“These are global companies, and the reason they have become trillion-dollar companies is that they’re really good at taking complex data and doing smart things with it,” says Bob Wachter, the chair of UCSF’s Department of Medicine and, on the strength of edifying tweeting, San Francisco’s unofficial town crier for the pandemic. “If these companies were taking this seriously, that got me to sit up and take notice, and it got others to sit up and take notice.”

It didn’t hurt that city officials were on the same page. On March 2, the very day Twitter encouraged all its employees to work from home—and within hours of a tweet from New York mayor Bill de Blasio encouraging his constituents to “go on with your lives + get out on the town”—Breed encouraged San Franciscans, via Twitter, to “prepare for possible disruption from an outbreak” by keeping medications on hand, making plans for childcare if schools closed or parents got sick, and caring for any family members who fell ill.

Around the same time, Colfax called Diane Havlir at UCSF to ask her and a few colleagues, including an epidemiologist named George Rutherford, yet another veteran local AIDS researcher, to create an informal advisory group—keeping an eye on emerging science and passing along what they learned. The group convened for the first time on March 6. From then on, information about the virus flowed directly from some of the world’s leading infectious-disease epidemiologists through Colfax to a mayor in charge of every city and county agency.

The day of the advisory group’s first meeting, Breed and Colfax urged everyone over the age of 60 to work from home, businesses to freeze nonessential employee travel and large in-person meetings, and all concerts and conventions to be canceled. In days to follow, as news broke of a terrifying outbreak at a nursing home in Washington state, Breed and Colfax issued increasingly restrictive public health orders: banning most visits to the enormous Laguna Honda Hospital, a city-run nursing home with 780 residents; ordering deep cleaning of single-room-occupancy hotels where residents often lived in crowded conditions; and shutting down the entire San Francisco Unified School District.

Colfax also convinced Breed to put a moratorium on gatherings—first of more than 1,000 people, then of more than 100 people. “They kept coming to me with arbitrary numbers to reduce events to,” Breed told me. “I was like, ‘What are we doing here? What is your medical advice for what makes the most sense?’” On Friday, March 13, she says, “I got aggressive with Doctor Colfax and said, ‘We need to shut the city down.’” Breed says she reached out to mayors of neighboring cities with the expectation of issuing a joint shelter-at-home order sometime the following week. The next day, though, a Saturday, Santa Clara County, just down the peninsula, reported an alarming acceleration in cases: from 71 to 227 in just five days.