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If your child has asthma, the tubes in his or her lungs become swollen and narrow. This makes it difficult to breathe.

Even in very mild asthma, inflammation (swelling) occurs in the airways. Preventing inflammation in the lungs with drugs helps to keep asthma under control.

• About 1 in 7 children in the United States are diagnosed with asthma at some time during their childhood.¹
• Up to three-quarters of children with asthma grow out of it.²
• Children with mild asthma are most likely to grow out of it.³
• There are lots of medications that can help prevent symptoms and ease them when they happen.
• You can also reduce asthma symptoms by learning what triggers your child's asthma and avoiding these if you can.

To understand what happens in asthma, it's useful to know how the lungs work and what they do.

The lungs sit in the center of your chest, behind your ribs. Their main job is to get oxygen from the air into your body. Your blood vessels carry oxygen from your lungs to the rest of your body. Every cell in your body needs oxygen to work.

• When you breathe in, air goes into your lungs.
• The main tube that goes from your throat to your lungs is called your windpipe (or trachea).
• Your windpipe divides into two tubes called bronchi (each individual tube is called a bronchus).
• Inside each lung, the air moves down a network of tubes called bronchioles.
• At the end of each tube are tiny sacs (called alveoli) surrounded in blood vessels.
• Oxygen in the air passes through these sacs and into the blood vessels.
• Carbon dioxide passes the other way back into your lungs. This is a waste product that you get rid of when you breathe out.

If your child has asthma, they can't always breathe normally. The tubes inside their lungs get narrower, so they can't get enough air in and out of their lungs. This can make your child gasp for breath. Or it might make them cough or wheeze.

To learn more, see What are the symptoms of asthma in children?

Three things happen in the lungs to make the air passages narrower:

• Muscles in the wall of the airways get tighter
• The walls of the airways become swollen
• Mucus is released into the airways, partially blocking them.

We're not exactly sure why some children have asthma and others don't. But two things seem to be important:

• The genes children inherit from their parents (asthma often runs in families)
• An infection or things in the air. For example, your child might have first got asthma when they had a cold or chest infection. Or they might be allergic to certain things in the air. These things are called allergens. Common allergens are pollen, house dust mite droppings, mold, and bits of fur and skin from pets.

After your child has asthma symptoms once, their airways are extra-sensitive. Whenever your child comes into contact with a trigger, their immune system overreacts. This makes their airways swollen and causes the symptoms of asthma.

It might be that the thing that first gave your child asthma symptoms (for example, dog hair) always brings on asthma symptoms. Or you might find that other things give your child symptoms as well.

Things that can bring on your child's asthma symptoms are called triggers. It's useful to know what triggers set off your child's symptoms. If your child can avoid some of these triggers, they might be able to prevent asthma symptoms from happening. The most common triggers are:

• Allergens
• Smoke and air pollution
• Exercise
• Other illnesses (such as a cold).

Some children are more likely to get asthma than others. Things that increase someone's chances of getting a condition are called risk factors. The most important risk factor for asthma in children is having asthma in their family.

If a child has a parent or close relative with asthma, they'll be more likely to get it, too.⁴

Diseases run in families because of genes that are passed from parents to their children. But there is no single gene that causes asthma. It happens because of a combination of many different genes. Some of these genes change the way your immune system works. But we don't know how these genes work together to cause asthma.⁵

If you inherit asthma from your parents, you might also inherit eczema, hay fever or both.

There are other risk factors for children getting asthma. To learn more, see Who's at risk?

Many babies and young children get wheezy from time to time, especially if they've got a cold or are just getting over one. For children under about 5 or 6 years old, it can be difficult for doctors to know whether wheezing is caused by asthma or a virus. Your doctor will want to be sure about what's causing the wheezing so that your child doesn't take medications he or she doesn't need. And it's also important for doctors not to miss when children have asthma, as this can be dangerous.

Often, if a young child has had several bouts of wheezing, doctors give them a "trial" of asthma medications. Your doctor will check on your child every so often to make sure the treatment is the right one.

Many of the medications used to treat asthma have not been formally tested on young children. So, don't be surprised if your doctor is very careful about prescribing some drugs for your child. He or she just wants to keep a close eye on what's going on.