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Depression is an illness which makes a person have negative emotions for a very long time.

It is more than just feeling a bit sad or upset.

It is a normal human emotion to feel sad occasionally. But depression is a medical condition, where a person has negative feelings that will not go away and it affects their life.

Celebrities like Selena Gomez and Lady Gaga have spoken out about how they have struggled with depression.

At the end of the summer, Selena Gomez announced she would be taking a break from music to deal with problems with anxiety and depression, as they can sometimes come together.

She recently made a speech at the American Music Awards where she said she "had to stop" because of the effect it had on her.

Newsround spoke to an expert on mental health from the charity Mind, Stephen Buckley, to find out more about depression, what causes it and what you can do if you're worried that it's something that's affecting you.

Depression is an illness where a person feels negative for a long period of time and it affects their life. It is not just feeling a bit upset for a few days.

It is a medical condition and can affect people in different ways, as you can read about below. Some may have mild depression, while others might experience it more strongly.

Many people with depression also have symptoms of anxiety.

Stephen says: "Everyone feels a bit down and sad at times, but usually these feelings pass or are for a specific reason.

"If the feelings are interfering with your life and don't go away after a couple of weeks - or if they come back, over and over again - then it could be that you're experiencing depression."

There is no one explanation for why someone might become depressed.

It can happen to anyone, but often is the result of something sad or stressful which happens to someone, which ends up having a long-lasting impact.

It may not be obvious which one thing has caused it or it could be the result of more than one factor.

Stephen says: "People can become depressed for all sorts of reasons. Some common examples are problems at home, problems at school, bullying and experiencing negative feelings about yourself."

Depression affects people in different ways, depending on who they are and how strongly they are experiencing the illness. It will not be the same for everyone.

It can make someone feel sad, worthless and negative about life for a long period of time.

They might feel like they have failed or let people down, when actually they haven't.

It can change people's behaviour and make them feel constantly very tired. They might not be able to sleep or they might want to sleep too much and not leave their bed.

In the same way, they might lose their appetite or they might start to eat too much or eat unhealthily.

It can make concentrating difficult and the person might regularly feel restless and fidgety.

Depression can stop a person from enjoying life. They might not have the energy, confidence or motivation to do things that usually they love doing.

While many people feel negative from time to time, it is not a feeling that you should have to live with permanently.

The most important thing is that you go to speak to an adult you trust about it so that they can help you.

If you think that anything described in this guide might be affecting you, follow these tips below.

Don't beat yourself up. Many people struggle with their feelings, so you mustn't feel ashamed of it.

If you are feel like any of the symptoms above might be happening to you, you should always speak to someone you trust.

They can talk through with you what might be making you feel that way so you can try to work out where the problem is.

They may take you to speak to a doctor or therapist, whose job it is to help with problems like this.

It's extremely important to make sure you're staying healthy, which means eating good food and making sure you're getting exercise.

It's important you don't shut yourself away, as much as you might like to.

Talk to an adult you trust and make sure you are still getting out and spending time with friends and family, or doing activities.

Stephen says: "It might seem overwhelming at times but it is really important to tell someone you trust how you are feeling. This could be a parent, friend, teacher or school nurse. There is no right or wrong person to open up to, but it should be someone you feel comfortable talking to."

"If talking to someone you know feels more daunting, then there are phone lines and online forums which can also offer support."

"Remember no matter how small your problems might feel there is someone who cares and will want to help."