Avril Lavigne has revealed she'd "accepted death" when writing her newest single from her sickbed "during one of the scariest moments" of her life.
The 33-year-old singer contracted Lyme disease, which is spread by infected ticks, in 2014.
"I have decided to be truthful about my struggle, open and more vulnerable than ever before," she writes.
Symptoms of Lyme disease include tiredness and headaches.
It's the most common tick-borne infectious disease in the UK, Europe and North America, and the flu-like symptoms can affect some people for years.
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In the letter to her fans, Avril speaks about battling her condition and "turning that fight into music".
She also talks about the importance of informing people how serious Lyme disease can be.
"To be honest, part of me doesn't want to talk about being sick because I want it to all be behind me, but I know I have to.
"Not only is it a part of my life, I need to bring awareness to the severity of Lyme disease."
The star has spoken previously about her health struggle since contracting the disease.
In 2015, Avril told US celebrity magazine People that she was stuck in bed for five months and "thought she was dying".
"I felt like I couldn't breathe, I couldn't talk and I couldn't move," she revealed.
The singer also said it took months for someone to diagnose her with Lyme disease, after previously being misdiagnosed with fatigue syndrome and depression.
Lyme prevention resources will now be available on the Avril Lavigne Foundation website in order to help individuals affected "get the treatment they desperately need".
"I'm taking my life back into my own hands and doing what it is I know I am meant to be doing in this life time," Avril adds.
"Making music. And sharing my healing and hope, through my music."
"Thank you for waiting so patiently as I fought through and still continue to fight, the battle of my lifetime."
Head Above Water is Avril's first single from her new album. Her last album, Avril Lavigne, was released in November 2013.
What is it like to live with Lyme disease?
"Avril Lavigne says it feels a lot like you're dying. I can definitely identify with that," says Laurie Walmsley.
"At risk of being a bit melodramatic it is kind of like a living death."
Laurie was bitten by ticks in Dartmoor in 2016, and was diagnosed by doctors in June 2018. He also lives with ME.
He says that other symptoms of Lyme disease include nausea, insomnia and "feeling like my insides are on fire all the time".
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Laurie adds: "I've had to surrender most of what makes life worthwhile.
"All the things that you take for granted when you are well - being able to walk to work, having the energy and money to see friends and pursue hobbies."
He says that "it takes days to recover" from small things like visiting a friend or going for a short run.
"I've also had very strong mood swings, and had very deep depressions on certain days.
"It does just consume your life."