As coronavirus outbreaks are slowly brought to heel in many places around the world, the US is among a handful of countries facing a surge of new infections. More than two dozen states are now seeing increases in new cases over the last 14 days. Of these, Texas, Florida, Arizona and California have emerged as the country's latest virus epicentres. But while cases are clearly rising, state leaders and health experts are divided on the cause. Here's a look at these four US hotspots, the facts and figures raising alarm, and the theories that may help explain each surge. What about testing? First, it's important to note that across the US, more efficient testing has played some role in the climbing case count. The number of Covid-19 tests being administered now is nearly double what it was in April and May. But the positive test rate tells us that testing can't explain away the rise. If lots of tests are being conducted and the spread of the coronavirus has been reduced, then the positive case rate would fall in tandem. The World Health Organization says that states should have a positive case rate at or below 5% for two weeks before they loosen restrictions on movement. Even with testing success stories, it's clear that the southern and western US are seeing a particularly sharp spike in infections and their rate. As of 30 June, Texas, Florida, Arizona or California all fall under that category - and all fail to meet the bar.