The difference between isolation vs quarantine is who gets separated. Isolation separates people who are sick from people who are not sick. Quarantine restricts the movement of people who were exposed to an infection but are not sick, to see whether they become ill. Those are the definitions used by CDC (page updated 15 May 2024), and they match the wording of WHO's International Health Regulations.
What is the difference between isolation and quarantine?
Isolation is for people who are ill. Quarantine is for people who might become ill. Both keep people apart to stop an infection spreading, but they act at different points in the chain.
The International Health Regulations (2005), the treaty that sets the rules for how countries handle health risks that can cross borders, define both terms in Article 1. Isolation means separating people who are ill or contaminated from others to prevent the spread of infection. Quarantine means restricting the activities of, or separating, "suspect persons who are not ill". In the same article, a suspect person is someone who has been, or may have been, exposed to a public health risk.
CDC uses the same split. Its page on legal authorities says isolation separates sick people from people who are not sick, while quarantine separates and restricts the movement of exposed people to see if they become sick.
Isolation and quarantine compared
| Who | Purpose | How long | Plague example | |
|---|---|---|---|---|
| Isolation | People who are sick, including those not yet confirmed by a test | Keep a known source of infection away from others | Until doctors judge the person is no longer infectious | A patient with suspected pneumonic plague cared for in hospital under droplet precautions |
| Quarantine | People who were exposed but have no symptoms | Watch whether they fall ill, and keep them apart while that is unknown | Generally the longest incubation period, counted from the last exposure | Close contacts of a pneumonic plague patient being watched for symptoms |
If a person in quarantine develops symptoms, they move into isolation and are tested. That is the point of quarantine: it catches new cases early, before they can pass the infection on.
What is quarantine, and how long does it last?
Quarantine is a waiting period for people who were exposed. Its length is generally set by the incubation period, the time between infection and the first symptoms. A 2020 article in the Canadian Medical Association Journal describes quarantine as keeping healthy, exposed people apart for the duration of the incubation period. In practice, health authorities usually work from the longest incubation period they expect for that disease and count from the last day of exposure, because someone who is still well after that point is unlikely to have been infected.
That is why quarantine periods differ from one disease to the next. For plague, the published incubation periods are short:
WHO's plague fact sheet (updated 29 September 2026) says people infected with plague often develop symptoms after one to seven days, and that incubation of the pneumonic form can be as short as 24 hours. CDC's plague symptoms page (updated 15 May 2024) gives 2 to 8 days for bubonic plague and says pneumonic plague can start as soon as 1 day after breathing in the bacteria. The exact quarantine period in any real event is set by the health authority handling it, so this page does not suggest a number of days.
How long does isolation last?
Isolation is not tied to a calendar. It lasts as long as the patient is considered able to infect others, which depends on the disease and on how treatment is going.
For pneumonic plague, the form of plague that can spread between people, CDC's guidance for hospitals (updated 15 May 2024) keeps patients with suspected or confirmed disease in isolation, under standard and droplet precautions, until treatment has had time to work and they are improving, or until plague has been ruled out. How hospitals set up that kind of isolation room is explained in transmission-based precautions. The same CDC guidance notes that being isolated can leave patients anxious or feeling stigmatized.
Who decides on quarantine and isolation?
Health authorities do, under their own laws. In the United States, CDC says federal isolation and quarantine are authorized only for a list of diseases set by executive order of the President, and plague is on that list alongside cholera, smallpox, measles and others. States have their own laws, which vary, and in some states local health authorities carry them out.
How do contacts end up in quarantine?
Quarantine only works if health workers know who was exposed. Finding those people is the job of contact tracing: listing everyone who was near a patient, sorting them by risk, and following them up.
For plague, WHO's fact sheet lists both steps among the core measures in an outbreak. Patients with pneumonic plague should be isolated so they do not infect others, and their close contacts should be identified, monitored and offered preventive treatment. Which medicines are used, and for how long, is a decision for doctors and health authorities, so this page does not list them.
Quarantine and isolation in the Irkutsk case
The suspected plague case in Irkutsk, Russia, in October 2026 shows how the two words are used in practice. In opening remarks on 7 October 2026, the WHO Director-General said Russia had replied to WHO that no case of plague had recently been reported in the Irkutsk region and that it had placed about 200 contacts in quarantine. That fits the definition above: they were people who might have been exposed, not patients.
Rospotrebnadzor, Russia's health watchdog, said on 7 October that medical monitoring was complete for more than 90% of contacts, as quoted by CNN and Euronews. Russian state media reported on 8 October that quarantine measures in the Irkutsk region had been lifted, according to the Kyiv Independent. WHO's Disease Outbreak News report of 10 October later gave Russia's account: contacts were kept under medical observation in healthcare facilities or at home for seven days, observation had ended by 9 October, and quarantine measures were put in place at the hospital where the worker was treated but at no other health facility. WHO had not received information identifying the cause of death when this page was checked. For the full timeline, see what happened in Irkutsk.
Why is it called quarantine? Where 40 days came from
The word comes from the Italian quaranta giorni, meaning 40 days. CDC's history of quarantine (updated 15 May 2024) says the practice began in the 14th century to protect coastal cities from plague: ships arriving in Venice from infected ports had to sit at anchor for 40 days before landing. A 2013 etymology note in CDC's journal Emerging Infectious Diseases ties the practice to European port cities during the Black Death, the plague pandemic of the mid-14th century.
The first recorded rule was shorter. According to that 2013 note, the Great Council of Ragusa, today's Dubrovnik in Croatia, set a 30-day separation period, the trentino, in 1377 for visitors from plague areas. Over the following decades other cities adopted the practice and extended it to 40 days. A 2013 history of quarantine in the same journal adds that Venice opened the first permanent plague hospital, or lazaretto, in 1423.
Why 40 days? The 2013 history says it is not known. It lists several theories, from medical ideas going back to Hippocrates to the 40-day periods in biblical stories, and notes that 40 days was longer than plague's incubation period. Modern quarantine keeps that logic but uses each disease's known incubation period instead of a fixed 40 days.
PlaguePrimer explains what these measures are and why they are used. It cannot tell you whether you need to quarantine or isolate. If you think you have been exposed to an infection, contact a doctor or your local health department, which sets the rules where you live. See our medical disclaimer.
