The number of coronavirus cases in the U.S. crossed the 50,000 mark on Tuesday, nearly doubling over the course of two days, according to John Hopkins University’s tracker. At least 606 people with the virus have died. The numbers of those with the virus have increased rapidly from about 25,000 on Saturday to 32,000 on Sunday to 50,206 on Tuesday afternoon. New York, the nation’s epicenter of the virus, accounts for more than half of the country’s cases. More than 90,000 tests had been done in New York by Tuesday, resulting in 25,665 positive cases, of which 3,234 were hospitalized. New York Gov. Andrew Cuomo said the increase was no longer akin to a freight train but a “bullet train.” Dr. Deborah L. Birx, the White House coronavirus response coordinator, warned on Monday that the percentage of New York’s population infected with the virus was five times higher than anywhere else in the country.