WASHINGTON, Oct. 4 (CGTN) - Nine U.S. states have recorded increases in COVID-19 infections over the past seven days, as cooler weather has led to more indoor activities.

According to Reuters, there were nearly 50,000 new infections on just Saturday alone across the country.

"It's well known that many respiratory viruses have seasonality," said Akiko Iwasaki, professor of immunology at Yale University. "And so, I wouldn't be surprised if there's actually elevated transmission during the winter months from COVID."

Health experts warned that colder temperatures will lead people to spend more time inside, which would promote the spread of the coronavirus. Also, they point out low humidity can increase the risk of airborne transmission.

To help contain the virus, New York City Mayor Bill de Blasio said on Sunday he plans to close non-essential businesses as well as schools in nine neighborhoods, starting on Wednesday. But the lockdown measures would require the governor's approval.

Some nursing homes and long-term care facilities in northern Wisconsin and Michigan have started to ban most visits similar to what they did earlier this year.

To reduce the spread of the virus, Iwasaki also recommends that people use humidifiers to moisten the air at home and offices.

"Another thing [a mask] does is it warms the nose and moistens the respiratory tract. So, I think masks are a great idea for multiple reasons, just even to boost this moisture inside the respiratory tract to better fight off the infection," Iwasaki added.