The 30-year-old "Rock n Roll" and "Girlfriend" singer revealed in a People magazine interview published on Wednesday that she has been battling the ailment, which had left her bedridden for five months. Foster, 51, suffers from a chronic version of the illness, which began as a bacterial infection obtained from a tick bite, for more than three years.
"Love my friend and fellow Lyme warrior @AvrilLavigne for sharing her story," she tweeted. "#chroniclymedisease #WeMustFindACure #AffordableForAll."
Foster has occasionally posted updates about her own battle on her Real Housewives blog. She published a new post on Wednesday.
"The path forward isn't completely clear," she wrote. "As I turn each corner, new obstacles arise, but I am a determined warrior, and even though this has brought me to my knees, I know God often uses our deepest pain as the launching pad of our greatest calling. So even though my life may not be perfect for now, I try to find a blessing in every day. Watching my children grow and establishing themselves into the world is my greatest joy and drive to continue to fight until I find my cure."
Foster, a Dutch American former model, revealed that due to her illness, she was unable to be there in person to watch her model daughters Gigi Hadid and Bella Hadid walk the catwalk at Tom Ford's autumn / winter 2015 fashion show in February in Los Angeles, where she lives. The event marked the runway debut for Bella, 18.
"I've waited my whole life for that exciting moment, but while I couldn't be there in person, at least today's technology allowed me to live-stream the show from bed," she said. "I've cried my eyes out for missing so many precious moments as their careers take off, while I've been forced to watch from the sidelines with great pride, inspiration and hope for a front row seat one day in the near future."
If diagnosed early, people who contract Lyme disease are usually cured by antibiotics. Up to 20 percent of patients continue to experience symptoms, which can include fatigue, muscle pains, sleep disturbances and cognitive difficulties, despite receiving the treatment. These people are often diagnosed with Post-treatment Lyme Disease Syndrome (PTLDS), also called Chronic Lyme Disease, according to the Centers for Disease Control and Prevention (CDC).
Foster had said in January that her ailment rendered her unable to "read, write, or even watch TV."
"Throughout this journey, I have finally, after 51 years, met the true core of my being," she wrote in her latest post. "I understand pain and suffering in a dimension I never knew existed. I am finding my own strength at its best, yet in the worst time of my life. I am getting to the most raw and vulnerable part of me and it has changed my perspective towards life and human beings for the rest of my life."
Foster had also discussed her Lyme disease battle on the recent Real Housewives of Beverly Hills season 5 reunion show, revealing that she had undergone treatment in South Korea.
"It was nice to see all the girls, even though, for obvious reasons, I have been extremely disconnected and pretty much living on a different planet for now," she wrote in her blog. "My overall take on the season is that we must all try to be kinder than necessary, because those hardest to love need love the most."