Gigi Hadid and Zayn Malik are back on, but she isn't saying a thing on Twitter or Facebook.

Although the 21-year-old model recently surfaced on Twitter for the first time since her and Malik's reunion was confirmed, she chose not to address the ups and downs of her relationship. Instead, she applauded her mother, Yolanda Foster, who recently wrote a book about her struggles with Lyme disease.

Yesterday, the cover art for Believe Me: My Battle with the Invisible Disability of Lyme Disease was released, and the Real Housewives of Beverly Hills star spoke out about what fans can expect when her publication hits stores early next year.

"I am determined to find a cure affordable for all," Foster told PEOPLE Magazine on Wednesday. "And it has been my dream to write a book to share all the treatments, knowledge, and spiritual growth I have acquired these past five years. As I've said previously, it's my Dutch foundation that gives me the balls and courage to share my raw and unedited story with the world! By sharing my journey in this book, I hope to offer a voice and encouragement to people who are ill and find themselves struggling while uncovering the mystery of chronic disease."

At the end of last year, as Foster was honored for raising awareness at a Lyme disease gala, Hadid spoke to Entertainment Tonight about being the only one of Foster's three children who does not suffer from the illness.

"I'm the only one in, of my siblings, my mom and my family that hasn't been affected by Lyme disease," Hadid said. "It's been really hard for me because I'm the only one that doesn't really understand it."

"What's been hardest for me is just trying to find a way to be their hope and kind of be a strength for them even with the disease that I can't understand fully because I haven't been through it," she added.

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Gigi Hadid, Zayn Malik