U.S. coronavirus cases are rising again, driven by rapid transmission in Midwestern states and sparking fears that a forewarned wave of infections this fall and winter has begun.

For almost a month, new U.S. cases have been trending upward. Since Saturday, more than 20 states have hit a new high in their seven-day average of case counts, and more than half of those states set records again on Tuesday.

The rising numbers are especially concerning because they set the stage for an even greater surge this winter when the virus will be helped by drier conditions and people spending more time indoors. The upward trend comes before the increased mingling of people expected to arrive with Halloween, Thanksgiving and Christmas.

The virus has become especially rampant in Midwestern states after dominating U.S. coastal and urban areas in the spring.

It is unclear what factors are driving the recent increase — whether it is the long-feared winter effect already taking place or the resumption of business and schools, or simply fatigue and people letting down their guard on social distancing efforts.

Because of day-to-day fluctuations in the reporting of cases, experts often look at the seven-day average of case counts to accurately spot trends.

In 40 states, cases are higher when compared with the week before.

Indiana, Minnesota and North Dakota have set a new average high for cases each of the past eight days. More than a dozen other states have set new average highs in recent days.

“A lot of the places being hit are Midwest states that were spared in the beginning,” said William Hanage, a Harvard University infectious-diseases researcher. “That’s of particular concern because a lot of these smaller regions don’t have the ICU beds and capacity that the urban centers had.”