Spring is here, which means if you suffer from hay fever, you’re about to endure months of sneezing and watery eyes. But why?

It’s thought that between 10 and 30 percent of adults suffer from hay fever (allergic rhinitis), and up to 40 percent of children. For many, symptoms improve as you get older, with about half of people reporting improvement in later life. In about 10 to 20 percent of people, symptoms eventually disappear completely.

Hay fever is an inflammation of your inner nose, which can be caused by a number of allergens including pollen, dust, or even flakes of skin. As a result, it causes cold-like symptoms like sneezing, itchiness, and a runny nose. While symptoms are usually mild, they can be extremely severe in some cases, leading to sleep problems and other issues.

Most people experience hay fever in different ways. Depending on what allergens you are allergic too, such as a particular pollen or even fungal spores, it may only affect you for a few months at a time during a particular season. For others, they can have hay fever all year round.

The cause of hay fever is our bodies producing allergic antibodies to the proteins found in various allergens. These sit in our immune cells on the outside of our body and when we inhale more allergens, they burst and release something called histamines.

This is a chemical that causes blood vessels to expand, usually protecting the body. But if the body gets confused and mistakes something harmless like pollen to a threat like bacteria, then it can produce histamines by mistake – leading to the various symptoms of hay fever. Fortunately, there are ways to stop this, notably antihistamines.

So why do some people develop hay fever and not others? Well, no one knows for sure, but there are a number of theories.

One idea is that it might be hereditary, a genetic trait that runs in families. Most people that have it are likely to have parents that have it too. Somewhat bizarrely, studies have found that the more siblings you have, the less likely you are to have it.

Another idea is that it might be due to your immune system being sort of out of practice. If you weren’t exposed to many parasites or microorganisms as a child, say by not working on a farm, then in later life, your body is more likely to mistake harmless things like pollen for bacteria.

If you’ve managed to avoid it so far though, don’t celebrate just yet. Adults can appear to develop hay fever in later life, even if they’ve shown no signs of it before.