As the coronavirus has spread across the United States, killing hundreds of people and sickening tens of thousands more, comprehensive data on the extent of the outbreak has been difficult to come by.

No single agency has provided the public with an accurate, up-to-date record of coronavirus cases, tracked to the county level. To fill the gap, The New York Times has launched a round-the-clock effort to tally every known coronavirus case in the United States. The data, which The Times will continue to track, is being made available to the public on Friday.

Individual states and counties have tracked their own cases and presented them to the public with varying degrees of speed and accuracy, but those tallies provide only limited snapshots of the nation’s outbreak. A publicly available tracker from the federal Centers for Disease Control and Prevention, updated five times a week, includes only state-level data. Other entities have made efforts, including a notable one by Johns Hopkins University, to track cases worldwide or within the United States.

In late January, not long after the first known case was reported in Washington State, The Times began tracking each known U.S. case as counties and states began reporting results of testing. Such testing, which had been delayed, gradually became more widely available. For the last eight weeks, a team of Times journalists has recorded an array of details — locations, dates, ages and conditions, when possible — about newly confirmed cases reported by state and local officials.