Ashley Olsen has been diagnosed with Lyme disease.

The 28-year-old fashion designer is said to be battling the debilitating bacterial illness, which is caused by tick bites and causes pain in the sufferer's skin and joints and problems with the heart and nervous system, due to bites she received from the parasitic arachnids several years ago.

Mary-Kate Olsen (L) and Ashley Olsen (R) pictured here on the Met Gala red carpet on May 4th 2015. Photo / Getty Images
Mary-Kate Olsen (L) and Ashley Olsen (R) pictured here on the Met Gala red carpet on May 4th 2015. Photo / Getty Images

A source close to the actress told America's OK! magazine: "She's really sick. It's gotten worse. She was diagnosed in the very late stages so early detection measures weren't options for her. She's really going through it."

The illness has taken a physical toll on Ashley - the twin sister of Mary-Kate Olsen and older sibling of Avengers: Age of Ultron star Elizabeth Olsen - in recent months and has affected her ability to work.

Mary-Kate Olsen and Ashley Olsen attend 2014 World Of Children Awards in New York City. Photo / Getty Images
Mary-Kate Olsen and Ashley Olsen attend 2014 World Of Children Awards in New York City. Photo / Getty Images

The source added: "When she does come to work, she looks haggard and dishevelled. And she's often moody. Ashley's having a very rough time right now."

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Pop singer Avril Lavinge has also been suffering from Lyme disease and recently admitted she thought she was "dying" when she was stricken down with the disease.

The Complicated hitmaker said: "I felt like I couldn't breathe, I couldn't talk and I couldn't move. I thought I was dying ... I had no idea a bug bite could do this. I was bedridden for five months."

- Bang! Showbiz