What will I have to do now?
As is currently the case, anyone who develops symptoms of coronavirus - a persistent cough, fever or a sudden loss of taste or sense of smell - will have to isolate for seven days and the rest of their household for 14 days.
The difference is that from Thursday, everyone with symptoms should ask for a test online or call to arrange a test by calling 119.
If the test comes back negative, everyone in your household can go back to normal.
But if the test comes back positive, the NHS Test and Trace team will get in touch - via text, email or phone call - to discuss whom you have come into close contact with.
Any of those contacts deemed at risk of catching the virus will be instructed by the NHS to go into isolation for 14 days, whether they are sick or not.
They will be tested only if they develop symptoms. The rest of their household does not have to isolate, unless someone becomes ill.
Those isolating will be eligible for statutory sick pay.
Anyone with symptoms - including the under-5s - can be tested for coronavirus.
What is a close contact?
Only people who have been "close" to an infected person will be contacted by NHS Test and Trace.
It will not be the case that one teacher testing positive would lead to the whole school going into isolation, for example.
Close contacts are:
- people you spend 15 minutes or more with at a distance of less than 2m
- people you have direct contact with - such as sexual partners, household members or people with whom you have had face-to-face conversations at a distance of less than 1m
The contact must have taken place between two days before and up to seven days after symptoms appeared.
What is the point?
At the moment coronavirus is being suppressed by lockdown restrictions which are applied across the whole of society.
The ambition is to pivot from lockdown for the many to isolation for the few. How far that can go will depend on the effectiveness of the scheme.
Baroness Dido Harding, the chairwoman of NHS Test and Trace, said: "[It] is designed to enable the vast majority to get on with their lives in a much more normal way... trading national lockdown for individual isolation."
Test and trace will also give far more detail on how and where the virus is spreading. Mr Hancock (Health Secretary) said this information could lead to local lockdowns to tackle flare-ups in towns, schools or workplaces.
Can the system cope?
- The government says it has 25,000 contact tracers ready to go and that it has plenty of testing capacity.
- The contacts of 2,000 people who have just tested positive for coronavirus will be chased on Thursday.
- However, the real pressure will be on the speed of testing.
- Prof John Newton, the national coordinator of Test and Trace said: "In order to be successful it requires rapid testing."
- Mr Johnson said on Wednesday there was now a target to get test results in less than 24 hours.
- There is a particular problem with home testing kits, which have to be posted out. Prof Newton said it would be "very difficult" to get those results in less than 48 hours.
- The NHS contact tracing app is not ready to roll out and was described by Mr Hancock as a "complement". He denied technical issues were delaying its roll out.