Tales of grim scenes have emerged from Italian and Spanish hospitals, where the sick have sometimes been left to crowd hallways and doctors and nurses have worked beyond the point of exhaustion to respond to the coronavirus pandemic.

The virus continued its march westward, and the United States overtook China in recent days as the country with the most reported infections. New York’s health care workers began to see a surge in cases and worried that they, too, would become overwhelmed.

Restrictions have begun to be put in place in Mexico, but shutting down the Latin American country, where many people live day-to-day off what they earn in the streets, is going to be harder than in Europe or the United States.

Meanwhile, virus prevention measures have taken a violent turn in parts of Africa. A day after South Africa’s three-week lockdown began, military troops raided a large workers’ hostel in the Alexandra township where some residents had defied the new restrictions.

For most people, the new coronavirus causes mild or moderate symptoms, such as fever and cough. For some, especially older adults and people with existing health problems, it can cause more severe illness and death. The vast majority of people recover.