Currently being researched. Many believe there's a genetic component, but it's unconfirmed. My doctor believes it's maybe genetics but always a trigger. He says whether you have the genes or not, some trauma triggers an immune response that starts the disease. He believed traumas could be injuries, serious illnesses like pneumonia, or even dietary triggers. From talking to other people, this rings true for me. A lot of us do have relatives with PR or similar conditions like RA, but some of us don't. A lot of us have history of injuries or being very sick when we were young. But it's all speculation and anecdote without more research.