As cases of the novel coronavirus surge throughout Europe, a French health advisor said the onset of cooler weather is partially to blame.

The French government on Thursday expanded curfews to cover some 60% of its total population, with French Prime Minister Jean Castex warning the situation was “very serious.” Spanish Health Minister Salvador Illa added the pandemic was “out of control” in many parts of the country and Belgian Foreign Minister Sophie Wilmes was put in intensive care to tackle COVID-19.

The European Center for Disease Prevention and Control estimates more than 5.3 million people across Europe have contracted COVID-19, compared with 7.7 million in India and 8.4 million in the United States.

Arnaud Fontanet, a scientific advisor to the French government, warned the virus is circulating more quickly. France by Friday will almost certainly pass the 1 million mark, becoming the second on the continent to reach the grim milestone after Spain. Fontanent said the spread was more or less under control during most of the summer months.

“And then there was one cold week in September and all the indicators went the wrong way again all over Europe,” he was quoted by the Reuters news service as saying. “The virus spreads better in the cold because we live more inside.”

Akiki Iwasaki, a Yale immunologist, told health news agency STAT that colder air and less humidity, typical for winter months in the Northern Hemisphere, means drier conditions that limit how mucus can protect against foreign invaders like the novel coronavirus.

“I do worry about this pandemic potentially getting worse this winter,” he said. “All the same kind of concerns that usually apply to other respiratory infections are the same with this virus.”

The silver lining perhaps is that hospital and other first responders have been here before, though that was at the height of the first wave in the spring when healthcare systems were strained to the max.

While outlining new social restrictions, the French prime minister warned of a “tough November.”

In the United States, meanwhile, a Reuters analysis finds 28 states have recorded daily-highs of COVID-19 cases in October alone. A report Wednesday from Columbian University pointed a finger directly at the federal government, saying the government’s response has been “an enormous failure.”