Between 6 million and 12 million people in the United States catch head lice each year.¹ However, it's difficult to know exactly how common head lice are because most people treat themselves at home and only see their doctor if this doesn't work.

Here are some theories about why more people get head lice today than in the past.

• People move around more than they used to, so head lice spread easily from one area to another.
• Children and adults are much more sociable than they used to be. Even babies mix with other babies these days, at day care and play groups. Twenty years ago, they would have spent most of their time at home.
• Treatments for head lice don't work as well as they once did because head lice are becoming resistant to them (this means the treatments don't kill all the head lice). So people can still have head lice after they think they've gotten rid of them.
• Parents are much more "hands-off" in the way they care for their children than they used to be. For example, a young child might be asked to brush his or her own hair, whereas 20 years ago the child's mother probably would have done this. Because of this, parents might not notice that their child has head lice for some time, so the head lice may spread to other children and adults.

• Children between ages 4 and 11 have the highest chance of getting head lice, because of their close contact with other children at school.² Adults between ages 24 and 36 are also very likely to get head lice because they often have close contact with children.²
• Girls have a higher chance of getting head lice than boys do.² Researchers think this is because girls are more likely to put their heads together when they're playing or working.
• You'll rarely get head lice from a stranger. It's more likely to be from someone in your family or your regular group of friends.³
• If you have long hair and have head lice, you're likely to have more of them than if you have short hair. This is because the head lice are less likely to find their way down long hair and onto someone else's head, so they stay where they are.³
• It's difficult to say if children will have fewer problems with head lice if they have long or short hair. But it's probably more difficult to get rid of head lice if their hair is long and thick.⁴