Poland has turned its largest stadium into an emergency field hospital. The numbers of Covid-19 patients in Belgium and Britain have doubled in two weeks. And doctors and nurses in the Czech Republic are falling ill at an alarming rate.

As new cases of the virus began to increase again across Europe last month, hospitals were initially spared the mass influx of patients they weathered earlier this spring. Some suggested that the virus had become less deadly, or that older, more vulnerable people would be shielded.

But a second wave of serious illness is here, new data released on Thursday shows, making it clear that the pandemic is still dangerous and that adherence to control measures over the next few weeks will be crucial in preventing hospitals from becoming overrun for a second time this year.

The number of Covid-19 patients in hospitals across the continent is still less than half of the peak in March and April, but it is rising steadily each week, according to data from the European Centre for Disease Prevention and Control. People across much of Europe — including larger countries like France, Italy, Poland and Spain — are now more likely to be hospitalized with Covid-19 than those in the United States.

Bruno Ciancio, the head of disease surveillance at the center, said he was concerned that some of the worst-hit countries now — including the Czech Republic, Poland and Bulgaria — were not as affected this spring, and may not have expanded their hospital capacity or intensive care units.

“The signals were all there in September,” said Mr. Ciancio. “At this point it’s very important that all member states prepare their hospitals to deal with the increase in demand that is coming.”

Hospitalization rates are a key measure of the pandemic’s severity. The rates rise and fall days or weeks behind the tallies of new infections. But infection figures depend heavily on each country’s testing capacity, while seriously ill people tend to enter hospitals whether they have been tested for the virus or not.

Europe’s current wave of infection is due in part to the relative normalcy it experienced this summer. Unlike the United States, where the epidemic rose to a second peak in July and a third peak this month, travelers moved around Europe, college students returned to campus and many large gatherings resumed, all while the virus kept spreading.

Now hospitals are scrambling to prepare for an onrush of Covid-19 patients, at a time when bed and intensive care capacity will already be under strain during the winter flu season.

In Poland, the government converted the country’s largest stadium into a temporary field hospital with room for 500 patients. Hospitals in France, especially in the Paris area, have started to postpone non-emergency surgeries, while others have called back staff on leave. More than one-fifth of Spain’s intensive care beds are occupied by Covid-19 patients, and in Madrid, that figure is closer to 40 percent.

And in the Czech Republic — where the current hospitalization rate surpasses the worst period in Britain — physicians are worried about a shortage of staff. “In some regions, about 10 percent of the medical staff is either already infected or in quarantine,” said Petr Smejkal, the chief of infectious diseases and epidemiology at the Institute of Clinical and Experimental Medicine in Prague.

Mr. Smejkal said the country also lacks specialty workers like respiratory therapists, and that most nurses are not trained to operate ventilators. “I am most worried about personnel, and keeping a safe ratio of doctors to patients and nurses to patients,” he added.

There is hope that no place will experience the level of death that Bergamo, Italy, New York City and Madrid suffered this spring. How the virus spreads is better understood now, and treatments have improved, giving sick people a better chance of survival. Testing has expanded across Europe, allowing countries to identify outbreaks earlier, when they are easier to contain.

But it is unclear how successful those control measures will be, or if political resistance and collective exhaustion over new restrictions will make it harder to get the virus under control for a second time.

Deaths in most of Europe remain at a fraction of the levels seen in the spring. But they have ticked slowly upwards over the last several weeks, and they tend to lag hospitalizations by about a month. Experts say additional increases in deaths are likely over the next couple of weeks.