Cases began to rise in late August. Health officials say now is the time for a flu shot.
Flu season often starts in October. This year, cases rose in late August.
Wastewater data shows flu activity at levels often seen in December or January.
The CDC says early activity is higher than normal. Most of the country is still in the minimal range.
California, Washington, Hawaii, and Alaska saw the rise first. Flu often starts in the South.
H1N1 made up 96 percent of typed US cases in late September. It has spread each season since 2009.
An early start does not mean a more severe season. But the flu can still turn serious fast.
Adults 65 and older, young children, and pregnant women face higher risk. So do people with asthma or diabetes.
The CDC urges a flu shot for all people six months and older. The best time is by the end of October.
A shot trains your body to fight the virus. It needs a few weeks to build full protection.
The FDA approved mFLUSIVA in August. It is the first mRNA flu shot, for adults 50 and older.
The new shot may not be in stock everywhere. A normal shot beats no shot at all.
Read the full explainer: the early season, the new shot, and what comes next.
Get the full breakdown, key numbers, and what happens next.
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