What is the cause of rheumatoid athritis?

Genetics and enviro. Ra is an autoimmune disease that is partly genetic and is thought to be triggered by enviromental factors most of which are unknown.
Auto-immunity. Rheumatoid arthtiris is one of a group of poorly understood auto-immune disorders. A combination of heredity and environment, sometimes prompts the body to act agaisnt its own tissues resulting in diseases like rheumatoid arthritis, systemic lupus, thyroid disease, multiple sclerosis etc.