Northern Ireland has become the first part of the UK to launch an app for tracking and tracing coronavirus.
Named StopCOVID NI, it has several features for logging details of those experiencing symptoms of the virus.
The app will run alongside a phone-based contact tracing programme already in place and will identify those at risk of infection.
It has been released for download from Apple's App Store and on Google Play.
StopCOVID NI will be officially launched on Friday but became available to download on Thursday afternoon.
Within an hour of its release, hundreds of people had installed it on their mobile phones.
Northern Ireland Health Minister Robin Swann said that the more people that downloaded the app, the more effective a tool it would be in the local coronavirus response.
"This could be the most important thing you do all year," he said.
"It could prevent you from spreading the virus to people you care about."
How does it work?
After a positive Covid-19 test result, a person will receive a unique code by text message.
That message will invite the person to enter the code if they use the app.
Entering the code will trigger a "Bluetooth handshake", allowing the app to notify any other user who has been nearby for long enough to be at risk of infection.
"There will be some people who won't be able to or won't want to use the app, and that's okay," said Dan West, the chief digital information officer at the NI Department of Health.
"The more people who do use it, the more protection this will provide to the whole community. We can say that for sure."
The app is intended for over-18s initially because of a conflict between data protection laws and the need for identifiable safeguarding consent.
The Health and Social Care Board is meeting the children's commissioner, the information commissioner's office and the Children's Law Centre to find a way through that.
BBC News technology correspondent Rory Cellan-Jones
There's been a lot of talk of the importance of getting contact tracing apps to work across borders - but between them Northern Ireland and the Republic have done it, a world first.
Their apps, built by the same developer, were designed to be compatible but what was vital was an agreement to share databases of the people who test positive for the virus.
What we still don't know is whether the app will work - as yet. There's not enough evidence from around the world to show that a technology-driven approach to contact tracing is effective.
But the NHS in England, which abandoned its first app to switch to the decentralised model used by StopCOVID NI, will be watching closely what happens in Northern Ireland.
What about the border?
The ambition was to have the two systems work in tandem, so information about contacts who need to be traced can be shared by both governments.
Both apps have been designed by the same company, Nearform.
On Thursday one more death was recorded in the Republic of Ireland, taking its death toll to 1,764.
An additional 85 confirmed cases were also reported to the Republic's Department of Health.
The contact tracing programme has been operational in Northern Ireland since mid-May.
It involves people with a positive test result being contacted by phone.
The people they have been within 2m of for 15 minutes or more are called and advised about isolating or being tested if they have symptoms.
The app is an add-on to that, to help with contact tracing, and alert those who may not be easily contacted.