Bella Hadid is so proud of her ''tough as nails'' mother Yolanda Hadid for battling Lyme disease.

The 20-year-old model - who, along with her brother Anwar, also suffers with the autoimmune disorder - took to Instagram to praise her 53-year-old mother for her new book 'Believe Me', which reveals how she has coped with her Lyme disease diagnosis.

Bella wrote: ''Mommy, I'm so proud of you for opening up about a very important and sensitive part of our lives. You didn't choose this path for yourself, nor did you choose to victimize yourself. Instead you continued to raise me, my brother and sister in the best way possible, wrote a book to help all of those suffering around the world, and slowly but surely found your happiness again. Whether that be with Lyme Disease, mental wellness, immune diseases or just being understood in general. I know how hard it was for me to see how much you struggled in the past few years and vice-versa, but to see others constantly judge you and doubt you while you were down was even worse. Hundreds of doctors appointments around the world, uncountable medications and IVs, remedies, the blood, the pain and tears... to help yourself and help people around the world find their voice, find a proper cure and a proper diagnostic.

''My whole life, I never saw you anything less than 'tough as nails' so every time you would look at me with tears in your eyes, pain in your bones, bruises on your body from all the needles, in the fetal position, and ask me when this would all be over... I would always say ''it will be over soon mommy'' without never really knowing the answer. Now I can finally say you are coming out of this stronger than ever. Thank you for being so strong and speaking up about our journey. I am so proud and I love you ''Believe me'' is available for Pre-order now on Amazon #LymeDiseaseAwareness @yolanda.hadid (sic).''

Bella's sister Gigi, 22, previously admitted how tough she has found it to cope with the stresses of Yolanda's diagnosis in 2012.

She said: ''You know it's hard, especially with something like Lyme disease, not a lot of people know about it, especially doctors so they don't really know how to help, medical wise.

''So it's really hard for me and my siblings to know how we could help, but I guess just being supportive is the best way we can do it.''