Quarantine is a state or place of isolation for a person or animal who may have come in contact with contagious diseases. The period of isolation lowers the chance that person or animal could transfer illnesses to others.
Quarantine isn't reserved for sick people only. People who appear healthy could spread a pathogen without ever knowing they were carriers, which is why travelers who appear healthy may still be quarantined, depending on where they are visiting from.
Novel coronavirus quarantine (in the U.S.)
For the COVID-19 pandemic, the CDC has recommended voluntary self-quarantine for individuals exhibiting symptoms and social distancing for everyone else, but a government-mandated quarantine is not in place. Many public and private institutions have taken the CDC's advice to heart and voluntarily cancelled events and issued work-from-home mandates in an effort to keep the rate of disease spread to a minimum.
If enough people participate in self-quarantine and social distancing, the number of COVID-19 cases is likely to remain at a manageable level for medical services. Health professionals call this "flattening the curve," because it keeps the number of cases over time below the maximum capacity of medical providers throughout the duration of the outbreak (see the graph below). At the time of publishing this article, the coronavirus pandemic is in full-force, and it remains to be seen if the U.S. can keep its 102-year streak of no government-mandated quarantines.
Brief history of quarantines
The concept of putting a sick person in isolation has been around for a very long time.
One of the earliest examples is found in the book of Leviticus, which recommends isolating people with leprosy. That evidence suggests that although people at that time had no knowledge of bacteria or viruses, they recognized isolation as a way to stop others from getting sick, according to a review published in The Virginia Tech Undergraduate Historical Review.
The practice of a quarantine as we're familiar with it likely began in the Middle Ages, according to the Centers for Disease Prevention and Control. In the 14th century, ships arriving in Venice from areas struck with the Black Death (bubonic plague) were required to anchor away from port for 40 days before docking. The Italians called it "quaranta giorni," or "40 days," which evolved into "quarantino." The 40-day quarantine was so effective that it became standard practice in Europe for the next 300 years.
In the United States, the Commonwealth of Philadelphia opened a quarantine station on the Delaware River in 1799 after the yellow fever epidemic of 1793 that killed around 5,000 people. In the 1830s, the mayor of New York City issued a quarantine for all ships and vehicles entering the city in an attempt to protect the city from a cholera pandemic. That quarantine wasn't too effective because numerous immigrants managed to find their way around the quarantine barriers and entered towns and cities throughout New England anyway.
During the Spanish flu of 1918 (the deadliest pandemic in history) health authorities in the U.S. and Europe recommended social isolation because they knew the flu-causing pathogen was spread through the air by coughing and sneezing. As such, several agencies banned public gatherings and closed public institutions, but how strictly the bans were enforced varied depending on the power of local health departments and perceived severity of the outbreak, according to a review published by Stanford University.
The Illinois and New York State Health Departments both issued mandatory quarantines for all ill patients, but that was also difficult to enforce. Entire military training camps were quarantined, which was a bit easier to enforce. At the same time, the American Public Health Association recommended that only patients with the most severe symptoms seek medical attention and those with mild symptoms remain at home.
Quarantines can be effective at minimizing the spread and risk of disease, but they're not always the best solution. The severe acute respiratory syndrome (SARS) epidemic of 2003 led to quarantines in many countries, sometimes when it may not have been necessary. For example, Canada quarantined about 100 people for every confirmed case of SARS, NPR reported. Toronto had only 250 probable cases, but 30,000 people were confined to hospitals or their homes — about the same as the number of people quarantined in Beijing, where there were 2,500 cases.
And quarantines remain difficult to enforce at a large scale. During the 2014 Ebola epidemic in Liberia and Sierra Leone, entire neighborhoods were put on lockdown and people were told they couldn't leave their homes. The civil unrest that resulted meant the quarantines led to the quarantines being lifted after three days. Doctors Without Borders, the medical organization that aided the fight against Ebola, later stated, "It has been our experience that lockdowns and quarantines do not help control Ebola, as they end up driving people underground and jeopardizing the trust between people and health officials."
U.S. government-mandated quarantines
The Public Health Service Act enacted by the U.S. Congress in 1944 gave the federal government legal authority to enact quarantines and respond to public health emergencies. (The U.S. Department of Health & Human Services is the agency responsible for declaring and responding to a public health emergency.)
The CDC's National Center for Emerging and Zoonotic Infectious Diseases now operates quarantine stations in 20 major U.S. cities and ports, with the goal of preventing pathogens from other countries from entering the U.S. The Division of Global Migration and Quarantine has the right "to detain, medically examine, or conditionally release individuals and wildlife suspected of carrying a communicable disease." The agency has a list of quarantinable diseases, which includes things like cholera, plague, smallpox and SARS.
The CDC also has the legal authority to issue mandatory quarantines at the state, local and tribal levels if it wishes. However, the last time the U.S. federal government issued large-scale quarantine orders was during the Spanish flu pandemic in 1918.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Kimberly has a bachelor's degree in marine biology from Texas A&M University, a master's degree in biology from Southeastern Louisiana University and a graduate certificate in science communication from the University of California, Santa Cruz. She is a former reference editor for Live Science and Space.com. Her work has appeared in Inside Science, News from Science, the San Jose Mercury and others. Her favorite stories include those about animals and obscurities. A Texas native, Kim now lives in a California redwood forest.