Contact tracing is how health workers find the people who were near someone with an infectious disease, tell them they may have been exposed, and watch them for a set period of time. The aim is to spot any new case early, before that person can pass the infection on. CDC's teaching guide on contact tracing (2024) describes it as quickly finding, assessing and managing exposed people so the disease does not spread further.
The method is old and works for many diseases. CDC's guide credits it as part of the strategies used against smallpox, Ebola and COVID-19. Plague makes a clear example, because CDC says infected droplets are the only way plague passes from one person to another, so the people to find are the ones who were close to a patient.
How contact tracing works, step by step
WHO's contact tracing Q&A (2017), written with Ebola in mind, splits the work into three steps: identifying contacts, listing them, and following them up. Adding what comes before and after, the process runs like this:
- Find the case. Someone is diagnosed with, or suspected of having, the disease. Health workers ask the patient, or their family, where they went and who was around them during the time they could have been infectious.
- List the contacts. Everyone who may have been exposed goes on the list. WHO names family members, work colleagues, friends and the health workers who cared for the patient. The health department then sorts the list by risk, using that disease's close-contact definition.
- Notify them. Each contact is told that they may have been exposed, what that means, what will happen next, and why early care matters if symptoms start. WHO says contacts should also get information on preventing the disease.
- Watch them, and quarantine some. Health workers check in regularly to ask about symptoms. WHO says high-risk contacts may need quarantine or isolation, at home or in hospital. For some diseases, plague among them, contacts may also be offered preventive treatment.
- Test and treat anyone who falls ill. A contact who develops symptoms is isolated, tested and treated. If they turn out to be infected, they become a new case, and the steps start again with their own contacts.
WHO's COVID-19 contact tracing Q&A (2021, now marked as no longer updated) sums up why the loop works: quickly finding, isolating and caring for cases, while supporting contacts through quarantine, can break chains of transmission.
What counts as a close contact
Not everyone on the list faces the same risk. A close contact is someone whose exposure was near enough, or long enough, to make infection plausible. There is no single definition: health authorities set one for each disease, based on how that germ spreads.
| Disease | How a close contact was defined | How long contacts were watched |
|---|---|---|
| Pneumonic plague | Close (under 6 feet, about 2 meters), sustained contact with a person or animal that has pneumonic plague, or direct contact with infected body fluids or tissues, per CDC's clinical care guidance (2024) | About 7 days (see below) |
| COVID-19 | Less than 6 feet from an infected person for a total of 15 minutes or more over 24 hours, per CDC (2022) | WHO advised 14 days of quarantine after the last contact (2021); countries set their own periods |
| Smallpox, during eradication | Household members, people who spent three hours or more with the patient, and people within 6 feet of a patient with a rash, per CDC's teaching guide | Checked daily for 14 to 18 days |
That plague row uses CDC's current clinical guidance (updated 15 May 2024). An older CDC fact sheet (2021) drew the line more narrowly: anyone within 6 feet of a plague patient while the patient was coughing up blood. WHO's plague fact sheet (updated 29 September 2026) refers to the "close contacts" of pneumonic plague patients without giving a distance. In any real event, the local health department decides who counts.
A close-contact rule belongs to one disease. For COVID-19, CDC defined a close contact as someone less than 6 feet from an infected person for a total of 15 minutes or more over 24 hours; that rule was written for a different germ and does not carry over to plague or anything else.
How long contacts are watched
Contacts are usually followed for the longest known incubation period of the disease, counted from their last exposure. The incubation period is the time between infection and the first symptoms, so a contact who stays well past it is very unlikely to have been infected. CDC's teaching guide describes tracers checking on close contacts throughout the incubation period.
Plague shows the logic. CDC's 2021 fact sheet puts the typical incubation period of pneumonic plague at 1 to 3 days, with a range of 1 to 6 days, and says a person exposed to infected droplets more than 7 days ago who is still healthy is very unlikely to develop infection. WHO's fact sheet says people infected with plague often develop symptoms after one to seven days, and that the incubation of the pneumonic form can be as short as 24 hours. That short window is why plague follow-up is counted in days, while COVID-19 follow-up was counted in weeks.
Contact tracing for plague
WHO's plague fact sheet lists contact tracing among the core steps for managing an outbreak. Health workers identify and monitor the close contacts of people with pneumonic plague and give them preventive antibiotics, and patients with pneumonic plague are isolated so they do not infect others. WHO adds that household members of people with bubonic plague should also receive preventive antibiotics.
Which medicines are used, and for how long, is a decision for doctors and health authorities, so this page does not list them. Anyone worried about their own exposure should talk to a doctor or the local health department (see our medical disclaimer).
Contact tracing, quarantine and isolation
The three terms often appear together but describe different things. Contact tracing finds the people who were exposed. Quarantine keeps exposed people who are not sick apart from others while they are watched; isolation separates people who are sick from people who are not sick, in CDC's words. Not every contact is quarantined: WHO's 2017 guidance says quarantine or isolation is required in some cases for high-risk contacts. The difference is explained in isolation vs quarantine.
Contact tracing in the Irkutsk case
The suspected pneumonic plague case in Irkutsk, Russia, in October 2026 is a current example. In opening remarks at the 7 October 2026 media briefing, the WHO Director-General said Russia had told WHO that it had placed about 200 contacts in quarantine. A day earlier, UN News reported a WHO spokesperson as saying that all of the patient's contacts had reportedly been identified and were being monitored, and that none had shown symptoms so far.
WHO's Disease Outbreak News report of 10 October gave Russia's later account: contacts were watched for seven days, in healthcare facilities or at home, with daily laboratory tests, all stayed free of symptoms and tested negative, and observation had ended by 9 October. WHO added a caveat that ties back to how long contacts are watched: because the cause of death has not been identified, it said it could not independently assess whether that monitoring period was long enough to exclude all plausible causes.
WHO had not received information identifying the cause of death when this page was checked. For the timeline and what each agency has said, see what happened in Irkutsk.
