Definition
Once one patient is confirmed, contact tracing is the work of finding the people who were close to that person during the time they could pass the illness on. Those people are told and asked to watch for symptoms. The goal is to know where they are before the illness moves from them to the next person.
One patient turns into one list
A patient has been confirmed with some illness. The first thing investigators ask is when the person started feeling sick. The next question is who they were with around that time.
There's a reason for that order. Investigators first need to pin down when the person could start passing the illness on. That sets how far back to look. Only people who were close during that period go on the list.
Not everyone the person crossed paths with gets written down, either. Each illness spreads in its own way, so each has its own rule for how close and how long someone has to be near the patient to make the list. An illness that needs a long face-to-face stretch and one that spreads after a short time in the same room produce lists of very different lengths.
Once the list exists, those people are told and asked to watch for symptoms. This is a list of people who haven't had anything happen yet, and that is what defines the work.
A few days gained before the illness is passed on
Most people on the list feel fine right now. Even if they've already caught it, nothing shows yet. And with some illnesses, they haven't even started passing it on.
Here you have to split the clock in two. The incubation period only tells you when symptoms show up. When someone starts passing it on is a separate question. What buys tracing its time isn't the stretch with no symptoms. It's the stretch when the person can't pass it on yet.
If you find them in that window, you can decide what to do before they hand it to the next person. In chain terms, you're dealing with the next link before it arrives. That's why how fast you find people matters more than how many you find. One day late, and the chain has already moved one link further. Then you have to widen the list beyond that.
In the spark picture, it's finding where the sparks landed before new fires start. But this comparison has to stop here. A person can throw sparks before any smoke shows, so waiting for smoke is too late. What you're waiting for isn't smoke. It's the moment sparks start flying from that spot.
To be a bit more precise
Contact tracing is the work of finding people. What happens after they're found is a separate matter. Isolation separates someone who can pass the illness on from other people. Ring vaccination gives vaccine to the contacts you found and the people around them, building a ring of immunity around the patient. The phrase "surveillance and containment," used in the smallpox eradication effort, covers both the finding side and the stopping side.
For this to work, you have to be able to trace who was reached. An illness that needs a long face-to-face stretch gives a short, clear list. An illness that spreads in passing leaves many people off the list. That's because the work relies on people's memory and whatever records exist.
There's one more condition. When the spreading clock starts ticking before symptoms appear, this work gets much harder. You begin from someone with symptoms, and by then the chain has already moved past the list.
This work alone doesn't make an illness disappear. It cuts the chain one spot at a time. Those cuts add up until a region reaches elimination, and in the end, eradication.
The spark picture holds up this far. It's the same idea: find where the sparks landed before new fires start. The difference is that a spot where a spark landed stays put, while a person keeps moving and meeting other people in the meantime.
Common misconceptions
Contact tracing means finding everyone who crossed paths with the patient.
Each illness has its own rule for how close and how long someone has to be near the patient to make the list. Only people who were close enough and stayed long enough for that illness to spread go on it.
Contact tracing and ring vaccination mean the same thing.
Tracing is the work of finding people. Ring vaccination is giving vaccine to the contacts you found and the people around them. In the smallpox eradication effort the two worked as a pair, and "surveillance and containment" is the phrase that covers both.
Contact tracing works well for any illness.
You have to be able to trace who was reached, and when an illness spreads before symptoms appear, the chain slips outside the list first. It only works if you find people faster than the chain moves.
Where you meet it
When it's confirmed that a measles patient was at a place, it means finding the people who were in the same space at that time, contacting them, and asking them to watch for symptoms.
When one patient was confirmed in an area with a smallpox outbreak, it meant tracing back through their household, neighbors, and visitors to the village to build a list.
In one sentence
Starting from one patient, it's finding the people who were close during the time that person could pass the illness on, so you know where they are before it moves to the next person.