'Putting my mouth where a million other mouths have been, just knowing all the bacteria that you carry in your mouth? Ucch!' The stars living with OCD...

Obsessive Compulsive Disorder is characterized by a series of irrational thoughts and fears, often labelled as obsessions, which lead to compulsive actions. Some of the world's biggest stars are living with OCD which can manifest itself in germophobia to being scared of stepping on a crack! Read on to find out which celebrities have OCD...

In an interview with radio station Z100, singer Katy Perry revealed her OCD manifests itself in a need for order. The ‘Last Friday Night’ hitmaker said: “I’m so OCD. I always want to put things in alphabetical order.”

Leo DiCaprio’s ‘Aviator’ character, Howard Hughes, famously suffered from OCD and was a reclusive germophobe in his later years. Portraying the real-life character led Leonardo to recall his own childhood OCD. He said: "I remember as a child, stepping on cracks on the way to school and having to walk back a block and step on that same crack or that gum stain. Let's just say it took me a while to get to set, having to step on tons of things.”

According to ABC, ‘Charlie’s Angels’ actress Cameron Diaz has a huge fear of germs and is obsessed with cleaning, to the extent of not using her bare hands to open doors. How does she handle that then? By using her elbows!

Repetition and numbers are related to our next celebrity, ‘Armageddon’ actor Billy Bob Thornton. He opened up about his OCD to Dateline, revealing that he is constantly doing math in his head. He said: "Certain numbers represent certain people. And I can't use that number in a certain circumstance. And then I have to use it in another circumstance. It exhausts you.”

Jess Alba told CosmoGirl that she wold freak out if she didn't turn off all the electric appliances in her home before going out or going to bed. She said: "I used to unplug every single appliance in my house. Or I'd double-check every door in my house to make sure it was locked at night. It was like a panic come over me and I had to do something, and once I did it, I was OK. ... It was really me needing to control something."

Former ‘Two and a Half Men’ star Charlie Sheen can't cope if he has untidy clothes or if sees strangers with creased shirt collars. In an interview with Dr. Oz, Sheen revealed that he simply can’t stand messy clothes. He will even fix a stranger's shirt collar if it isn't straight.

‘Deal or No Deal’ host Howie Mandel revealed in his memoir ‘Here's the Deal: Don't Touch Me’ that he fears germs and dirt. He will not even shake hands with people to avoid them transmitting germs to him. And he keeps his head shaved because it feels cleaner than having hair. Howie was even hypnotized on an episode of 'America's Got Talent' so he would shake hands with his fellow judges, but it was only a temporary solution. He said: "I was upset about it, and it's hard for me to watch. I didn't know. I was not prepared for that to happen. ... I was open to be hypnotized. I just didn't know what he was going to ask me to do. "As soon as [the hypnotist] snapped me out of it, my guard is back up. He didn't cure me."

The Oscar winner just can’t deal with mess nor hoarding. Speaking to The Daily Mail newspaper, she said: "I actually have a serious problem - obsessive compulsive disorder. I have a problem with cabinets being messy and people just shoving things in and closing the door. I will lie in bed and not be able to sleep because I'll say to myself, 'I think I saw something in that cabinet that just shouldn't be there.'"

It was David's wife Victoria Beckham who revealed details about her husband's OCD. Everything in David's life must match and must be ordered, even the items in their fridge, well three refrigerators! She said: "Food in one, salad in another and drinks in the third. "In the drinks one, everything is symmetrical. If there's three cans of Diet Coke, he'd throw one away rather than having three, because it has to be an even number."

There’s TWO specific things ‘Transformers’ star Megan Fox just can’t stand. She once told Allure that it is unthinkable to her to use a toilet without a seat cover. She said: “Every time someone uses a bathroom and they flush, all the bacteria is shot into the air." And Megan simply cannot use cutlery in restaurants because so many other people have eaten with those knives and forks. She said: "Putting my mouth where a million other mouths have been, just knowing all the bacteria that you carry in your mouth? Ucch! I have an illness, this is not OK anymore."