This is one of the most basic laid down practice of public health responses to a pandemic. It means literally tracking down anyone that somebody with an infection may have had contact within the days before they became ill.
It was central to the fight against Ebola, for instance. And always will be a fight against any infections that will occur. In west Africa in 2014/15, there were large teams of people who would trace relatives and knock on the doors of neighbours and friends to find anyone who might have become infected by touching the sick person.
Most people who get Covid-19 will be infected by their friends, neighbours, family or work colleagues, so they will be first on the list. It is not likely anyone will get infected by any ordinary one they do not know, passing on the street.
It is still assumed there has to be satisfactory exposure – originally experts said people would need to be together for 15 minutes, less than 2 metres distant. So the contact tracer will want to know who the person testing positive met and talked to over the two or three days before they developed symptoms and went into isolation.
The tracer would call the contacts and ask them how close they had been to the person with symptoms and establish whether they were low-risk or high-risk. If the latter, they would be asked to isolate themselves for 14 days at home.
The contact tracers would either call each day to check how they were or ask them to phone if they felt ill. If they developed symptoms, the tracers would start again, looking for their contacts in turn.
This process will continue as long as the fight of the pandemic is in battle. A contact tracer is originally a public health practitioner or selected from the local council. They wear protective types of equipment in their daily activities for them not to contract the virus themselves.