People often don't even realise they are in an abusive relationship.

It can be hard for others to understand why someone stays with an abusive partner.

It's often because of something called "trauma bonding," where you become addicted to the hormonal rollercoaster an abuser sends you on.

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Those who have never been in an abusive relationship struggle to understand how people remain in one for so long. If somebody was mistreating you, "why did you stick around?" they ask.

For survivors, this can be a really tough question to answer. The lucky ones escape, and stumble upon articles or books that give them the terms to be able to understand what happened to them, and thus describe their experience. Other times, though, this doesn't happen, and people might not even be aware they were in a relationship that could be classed as "abusive."

This is because we are conditioned to believe abuse is always physical. On TV and in films, we see characters who are obviously evil. They are violent to their partners, shout at them aggressively, or even murder them in a fit of rage. While this does happen, it's not a true representation of the abuse many others experience.

According to therapist Shannon Thomas, author of "Healing from Hidden Abuse," psychological abuse is insidious, and it occurs a over time like an IV drip of poison entering your veins.

It starts with an off-hand comment here, or an insult there, but often victims brush these moments off. This is because abusive people are great at pretending to be everything you're looking for in a partner, and they love bomb you with affection. Victims tend to believe this is the abuser's real self, and when the mask starts to slip more and more, they believe its "out of character" and it must be their own fault for making their partner angry.

People stay in these relationships partly because they are trying to win back the abuser's affection. However, Thomas told Business Insider that victims also become biologically attached to their abusers through something called "trauma bonding."

Read more: A marriage therapist says there are 4 good reasons to leave a relationship — and a really bad one

It's like an addictive drug.

It's a bit like becoming addicted to a drug. A psychologically abusive relationship is a rollercoaster, with punishment and then intermittent reinforcement of kindness when you "behave." This means the body is going through its own turmoil, with high levels of the stress hormone cortisol, paired with dopamine when given affection as a reward.

"You have this back and forth, and the body becomes addicted," Thomas said. "When we're looking for something that we want, that we once had, which is a connection with somebody, and they are playing cat and mouse where they are pulling it back and forth, then the body really does become dependent on having that approval."

This hormonal rollercoaster really takes its toll on someone's body. Victims might find they break out in acne, even though they've always had good skin. They might have chest pains. Thomas has said that in her practise she has even seen her clients develop autoimmune disorders.

"Their bodies start to shut down, and they start really struggling with chronic pain, migraines, and some arthritic type pains and conditions, and they just can't fight infections as well," she said. "The body really can only take so much stress."

Read more: Narcissists all follow the same patterns — here are some of the most common phrases they use

Victims stay in these relationships despite of the stress on their bodies, because often it isn't clear to them what the problems really are. Through gaslighting, control, and intermittent love, the abuser has their partner backed into a corner of self-blame and desperation of trying to win back the affection of the person they love.

Unfortunately, for many people, when they try to leave these relationships they are so bonded to their abuser that they return. Others don't try to leave at all, and are only freed from the clutches of the abuse when they are discarded.

An abusive relationship with a narcissist or psychopath tends to follow the same pattern: idealization, devaluation, and discarding. At some point, the victim will be so broken, the abuser will no longer get any benefit from using them. They may have totally bankrupted them, or destroyed their confidence, or worse, and they move on to their next target.

However, once they are gone, the victim — or survivor as Thomas calls them at this point — can finally start coming round to the idea they were abused. They can grieve, and finally see the damage that was being done, and realise it wasn't their fault.

That's when the healing can really begin, Thomas says, and the survivor can realise that they were targeted not because they were weak, but because they had so much to give.

Read more: Divorce isn't a failure, therapists say. In fact, it could mean the marriage was a success.

These are the signs you might be in a trauma bond with someone, according to Psych Central: