The catwalk star has been struggling with the condition since 2014, but kept her illness secret until her mother, model-turned-TV star Yolanda Foster, went public with the news at a charity gala in October (15).
Bella, whose sister is fellow model Gigi Hadid, has now spoken out about her condition, revealing it has completely changed her life as she had to give up her dream of competing in equestrian events at the 2016 Olympic Games in Brazil.
"It was a dark time (when I first got sick)... It affected my memory so I suddenly wouldn't remember how to drive to Santa Monica from Malibu where I lived," she tells Britain's ES Magazine. "I couldn't ride. I was just too sick. And I had to sell my horse because I couldn't take care of it."
However, giving up riding allowed her to take up modelling and forge a successful career on the catwalk instead.
"If I'd had the chance to succeed in riding, I might not have modelled," she adds. "I didn't know that I was going to get sick."
Since shooting to fame, Bella has found love with Canadian musician The Weeknd, and she admits he has been very supportive and "really good about me being sick".
Bella, who believes she contracted the tick-borne illness through her contact with horses, now makes sure she stays healthy by avoiding the modelling industry's party circuit.
"I used to be the last to go home at a party, now I'm the first..." she explains. "This was my first full (catwalk) season. I was so exhausted... I only did 13 shows... The show girls do, like, 12 shows a day. I only did one or two shows a day and I was like: 'I need to go to sleep'."