Ashley Olsen has reportedly been diagnosed with Lyme disease after suffering tick bites several years ago.
The 28-year-old fashion designer is said to be battling the debilitating bacterial illness - caused by bites from the parasitic arachnids - which causes pain in the sufferer's skin and joints and problems with the heart and nervous system.
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 apparently taken a physical toll on Ashley - the twin sister of Mary-Kate and older sibling of Avengers: Age of Ultron star Elizabeth - in recent months and has affected her ability to work.
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."