Arthritis is an umbrella term that simply describes joint inflammation. There are more than 100 types of arthritis, and they all can have different triggers and characteristics. As such, arthritis is one of the most common chronic conditions, affecting about 54 million Americans or nearly a quarter of all adults. But not all forms of arthritis are created equal. Some, like psoriatic arthritis, are related to immune disorders and can cause a lot of pain and disability.