U.S. coronavirus cases rise by nearly 50,000 in biggest one-day spike of pandemic

(Reuters) - New U.S. COVID-19 cases rose by nearly 50,000 on Wednesday, according to a Reuters tally, marking the biggest one-day spike since the start of the pandemic.

The record follows a warning by the government's top infectious diseases expert that the number could soon double to 100,000 cases a day if Americans do not come together to take steps necessary to halt the virus' resurgent spread, such as wearing masks when unable to practice social distancing.

In the first week of June, the United States added about 22,000 new coronavirus cases each day. But as the month progressed, hotspots began to emerge across the Sun Belt. In the last seven days of June, daily new infections almost doubled to 42,000 nationally.

Brazil is the only other country to report more than 50,000 new cases in one day. The United States reported at least 49,286 cases on Tuesday.

More than half of new U.S. cases each day come from Arizona, California, Florida and Texas, home to 30% of the country's population. All four states plus 10 others saw new cases more than double in June.

The daily increase in new cases could reach 100,000 unless a nationwide push was made to tamp down the fast-spreading virus, Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, told a U.S. Senate committee on Tuesday.

"We can't just focus on those areas that are having the surge. It puts the entire country at risk," Fauci said.

The rise in cases is not just the result of more testing. Hospitalizations are also skyrocketing.

Nationally, 7% of coronavirus diagnostic tests came back positive last week, up from 5% the prior week, according to a Reuters analysis. Arizona's positivity test rate was 24% last week, Florida's was 16%. Nevada, South Carolina and Texas were all 15%, according to the analysis.

Some of the recent increase traces back to Memorial Day holiday celebrations in late May. Health experts are worried about Independence Day celebrations this weekend, when Americans traditionally flock to beaches and campgrounds to watch fireworks displays.

(Writing by Lisa Shumaker; editing by Jane Wardell, Bill Berkrot and Leslie Adler)