In towns and cities around the world, the streets are eerily quiet. Shops, restaurants, and theaters are shuttered, schools are closed, and sports teams no longer play at all, much less in crowded stadiums. Many people have confined themselves to their homes, afraid to venture out, bewildered by the cause of these events, and fearful of what may come. On March 11, 2020 the World Health Organization declared the outbreak of a new, highly contagious virus dubbed COVID-19, officially a pandemic, affecting over 114 countries globally by mid-March of 2020.

The history of the world is filled with epidemics and pandemics caused by pathogenic bacteria and viruses. Viruses such as HIV, SARS, and influenza have sickened and killed millions. But except for the great influenza pandemic that erupted in 1918, killing as many as 100 million people, no infectious disease has spread so rapidly and disrupted every day life so dramatically as COVID-19.

Where did this disease come from and why is it spreading so rapidly? Many emerging diseases— infectious diseases not seen previously in human populations—arise when a virus is transmitted from animals to people. Influenza passes readily among pigs, birds, and humans, HIV originated in chimpanzees, and epidemiological evidence suggests that the SARS virus moved from a small mammal, the palm civet, to humans. Although the exact origins of COVID-19 have not yet been determined, it is likely that, similar to the SARS virus, this new virus derived from a small mammal found in China.

In most cases, an outbreak of a disease will stay local and not spread, but COVID-19 rapidly escalated into a deadly pandemic. Several factors contribute to rapid escalation. How a virus is transmitted from person to person is an important determinant in whether a disease will spread. A cough or sneeze from a COVID-19 infected individual will deliver droplets filled with virus into the air. These droplets can enter the noses and mouths of those nearby or can land on surfaces where the virus can remain infective for up to 24 hours, waiting for an unsuspecting hand to touch it and then bring their hand to their mouth, nose, or eyes. While people are the most contagious when they are the sickest, even asymptomatic individuals are thought to be capable of passing on the virus, making this virus even more highly contagious than influenza or SARS. And finally, air travel has contributed to carrying this virus around the world with unprecedented speed. Infected travelers may not just infect those sitting nearby, but perhaps more significantly, bring disease to lands far from wherever they originated.

Quarantine and isolation have been used effectively for centuries to prevent the spread of disease. In general, quarantine has been used to separate well individuals who have been exposed to disease before it is known whether they will become ill; isolation usually referred to the separation of sick individuals from the population to avoid further transmission of the disease. In today’s COVID-19 world these words have taken on broader meanings. Many individuals, ill or healthy, exposed to the disease or not, are quarantining and isolating themselves for their own protection and the greater good. The phenomenon of social distancing, a term previously unknown to most, is now common practice. This means people avoid large gatherings, dining out, and movie theatres. They work from home, attend classes through the Internet or radio, and hang out with friends and family via electronic devices. Hence, the eerily quiet streets.