Eleven days ago, Louisa County, Iowa — population 11,000 — had its first confirmed case of the novel coronavirus. The person who contracted the virus was in a high-risk group, over 60, but was self-isolating at home.

“While this is Louisa County’s first case, it may not be the last,” the county’s public health administrator said in a news release issued April 2.

It was not the last. According to data from Johns Hopkins University, the first case was joined by five others April 5. By April 7, there were 20 confirmed cases in the county. By Saturday, there were 70 cases, more than six confirmed cases for every 1,000 residents in the county.

With 79 cases as of this writing, Louisa County isn’t a hot spot in the sense that it’s comparable to what has emerged in New York City. But it’s a reminder that smaller-scale surges can and do erupt in various places across the country every day.

The interactive below shows county-level data from Johns Hopkins over the past few weeks. By default, it shows the change relative to the week prior in each location, the percentage-point growth in a county relative to where it was. Circles illustrated with red are counties where, on the indicated date, there were at least 50 confirmed cases. (The size of the circles is relative only to the day being shown, so the largest circle is the county with the biggest increase.)

Show per county.

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The government’s response to the pandemic is predicated to a significant extent on the eventual ability to limit and contain outbreaks that occur. Rescinding efforts to keep people apart (and, therefore, halt the spread of the virus) depends on the ability to ensure that the virus can’t emerge and spread in an unprotected community. Often, though, clusters of confirmed cases pop up fairly quickly.

On Saturday, for example, here were the places that had seen the biggest increase in cases relative to the prior week.

  1. Louisa County, Iowa: 6,900 percent increase to 70 cases.
  2. St. Francis County, Ark.: up 3,000 percent to 31 cases.
  3. Johnson County, Ga.: up 1,400 percent to 15 cases.
  4. Gage County, Neb.: up 1,150 percent to 25 cases.
  5. Oglethorpe County, Ga.: up 1,100 percent to 12 cases.

The increases are large because the number of cases is fairly small. A jump from 1 case to 11 cases is a 1,000 percent increase, but only 10 more infections.

Even places with relatively large numbers of infections saw big increases during that same period, though. Minnehaha County, S.D. — home of Sioux Falls — saw a jump of 440 percent, hitting 438 cases. By Sunday, it had seen more than 500 cases.

One thing to watch on the map above is the number of counties with red dots. Over time, there are more, especially near New York City. At times, the circles turn red after the size of the circles swells.

You can easily spot Louisa County, near the Illinois border in the eastern part of Iowa, in the data for April 10 and 11. By April 12, though, it dropped to third, behind Wilcox County, Ga., (near an outbreak in the southwestern part of the state) and Ford County, Kan.

Between those two counties, there were 35 confirmed cases by Sunday, a modest number. But context is key here. As of Friday, there had been only seven cases recorded between them.