SARS and MERS didn’t cause the same level of devastation that COVID-19 has largely because they aren’t as easily transmitted. Rather than moving by casual, person-to-person transmission, SARS and MERS spread from much closer contact, between family members or health-care workers and patients (or, in the case of MERS, from camels to people directly). These viruses also aren’t spread through pre-symptomatic transmission, meaning infected people don’t spread it before they have symptoms. Once people got sick, they typically stayed home or were hospitalized, making it harder for them to spread the virus around.

“By and large, except for a couple of mass transmission events, almost all of the transmission of SARS was within the health care setting, when you have an aerosol-generating event like intubating someone or dialysis,” said Stephen Morse, an infectious disease epidemiologist at Columbia University’s Mailman School of Public Health. “So basically, you could control SARS by improving infection control and prevention in the hospitals.”