This flu season is turning out to be so intense that the number of people seeking care at doctors' offices and emergency rooms has surged to levels not reported since the peak of the 2009 swine flu pandemic, federal officials said Feb. 9.

This year's vaccine is less effective against the strain of virus making the rounds. But there's another factor that gives aggressive viruses such as this one an extra punch in the United States: lack of access to paid sick time.

This index measures the cumulative number of flu-related hospitalizations per 100,000 people nationwide during flu season.

Cumulative flu-related hospitalizations per 100,000 people as of

Flu-related hospitalizations per 100,000 people each week as of

Flu level for week ending

This map shows the rate of flu-like illness in the most recent week of CDC data. The levels indicate how far each state is above its baseline level.






How the flu virus works

1. The genetic code for the flu is contained in eight strands of RNA. A protein on the virus binds to receptors on healthy cells in the airways and lungs, causing the virus to open and release its RNA.

2. The RNA moves to the cell nucleus, where it is incorporated into the cell’s machinery, directing the cell to make copies of the virus.

3. Another protein on the virus punches a hole in the cell, killing it and releasing the replicated virus.

4. The released virus either goes into the airway to find another cell to infect or it is ejected by a cough or sneeze and launched to find a new host.


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